Clean up serial_jtag and increase timeout

[bugadani]

cc #53 - this is a sufficient workaround for my application to not generate broken defmt frames.

I suspect the issue with lost bytes is that defmt calls write_bytes in rapid succession and the host may not have enough time to poll the interface between writes. This PR just tries to increase the timeout but there should be better ways to handle this that results in better FIFO utilisation.

[bjoernQ]

LGTM! Thanks!

Pretty much what I originally had in mind but back then it turned out it worked "good enough" with the very short timeout.

It shouldn't cause problems but if it does, we can still introduce features to select a different timeout