spi: clarify traits are for master mode.

[Dirbaio]

All dirvers out there assume the traitsare for master mode. However, there are some HALs
out there that impl the traits for SPI in slave mode, which will break when used with drivers.
(stm32f1xx-hal, stm32f4xx-hal, stm32l4xx-hal, probably more).

If we add SPI slave traits in the future, they should be a separate set of traits because the
API would look quite different:

• you'd want the API to give extra extra info about the transaction lengths: if you start a transfer with 256-byte buffer but the master only sends 10 bytes (sets CS low, sends 10 bytes, sets CS high),
  you'd want transfer to return, saying "I got only 10 bytes", instead of hanging waiting for the other 246 bytes.
• The bus/device split doesn't make sense in slave mode.

Also IMO the spi, spi_slave naming is fine even if inconsistent, since the vast majority
of uses are master mode. I wouldn't name the spi module spi_master.

[rust-highfive]

r? @ryankurte

(rust-highfive has picked a reviewer for you, use r? to override)

[vldm]

Hi there again, sorry if i annoy you with this spi slave :)

But wanting to clarify some some usage examples:

you'd want the API to give extra extra info about the transaction lengths: if you start a transfer with 256-byte buffer but the master only sends 10 bytes (sets CS low, sends 10 bytes, sets CS high),
you'd want transfer to return, saying "I got only 10 bytes", instead of hanging waiting for the other 246 bytes.

Not always you need to give feedback to developer. (Real world example https://img.jdzj.com/UserDocument/2017y/wenzihui/dn/zl7968.pdf)
Slave spi drived vfd display.
But in it's specification it enforce:
a) min CLK timing
b) count of bits

And it's responsibility of master to make sure that it transfer needed amount of bits, so slave just wait for FINISH(LAT in case of VFD) signal in order to understand that transfer is finished.

My thoughts is that BUS configuration code are very common for slave and master, and BUS usage code (transfer/read implementation) are almost same for them.

But there is lack of cancellation with feedback.
Example of use cases, that need cancellation:

• Handling "not enough bytes/bits" program error
• Handling timeouts
• Handling multi-master setup
• Handling any other interruption that should force transfer to abort

All of them are important on master too, not only on slave.

But SpiDevice is a trait that implement SS and concrete usage scenario, and there can be used to define master/slave logic.

[Dirbaio]

oops I had this accidentally marked as draft

[eldruin]

Looks good to me, thanks!
bors r+

[bors]

Build succeeded:

• ci-linux (1.54.0, x86_64-unknown-linux-gnu)
• ci-linux (stable, thumbv6m-none-eabi)
• ci-linux (stable, thumbv7m-none-eabi)
• ci-linux (stable, x86_64-unknown-linux-gnu)
• ci-linux-test (1.54.0, x86_64-unknown-linux-gnu)
• ci-linux-test (stable)
• fmt