tkey: gate CDC and FIDO handling on endpoint match, poll in readselect

[jthornblad]

Description

• Change readselect() to non-blocking (true) instead of blocking (false) to be able to process data that have been put into the fifo from other places than main.
• Guard the FIDO frame length check, read(), and fifo_hidmsg_add() behind (ep == IO_FIDO) && (available >= 1), since these previously ran unconditionally even when the received data came from IO_CDC
• Use discard() instead of read() to drop incomplete HID frames
• Replace the fifo_hidmsg_add() failure's return -1 with an assert, matching the other error-handling style in this loop

Type of change

Please tick any that are relevant to this PR and remove any that aren't.

• Bugfix (non breaking change which resolve an issue)
• Feature (non breaking change which adds functionality)
• Breaking Change (a change which would cause existing functionality to not work as expected)
• Documentation (a change to documentation)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have added a "Co-authored-by: x" if several people contributed, either pair programming or by squashing commits from different authors.
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)
• QEMU is updated to reflect changes