Fix issues with new async SPI changes that broke compiling for B_U585I_IOT2A

[multiplemonomials]

Summary of changes

In #199, I made a breaking change to the async SPI interface to require use of cache-aligned buffers. These are required for correct functionality when DMA SPI is being used, but unfortunately required heavy, breaking changes to the SPI API.

Turns out that when I did that, I broke stuff badly for B_U585I_IOT2A and its EMW3080B wifi module. Had to make two fixes:

• Some typos in SPI.h were causing the SPI::transfer(nullptr, 0, rx_buffer, len) overload to not work correctly. I fixed those and now it works as intended -- turns out you are supposed to accept std::nullptr_t and not std::nullptr_t *
• The EMW3080B driver has extensive infrastructure built on top of Mbed SPI, including its own network memory manager. I had to rewrite this part of the driver to use DynamicCacheAlignedBuffer objects to allocate the buffers instead of just creating them with malloc.

Impact of changes

B_U585I_IOT2A compiles again after #199!

Migration actions required

Documentation

None

Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

This was tested by @tim35ca on the forums

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Reviewers

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