chore(fatfs): Port to new mutex API

Using a bare FreeRTOS mutex is deprecated.  Replace it with the new
`struct mutex`.  This should increase stability of the module.  In the
process of switching, also remove the `EPIC_FAT_STATIC_SEMAPHORE` define
as it is no longer needed with the new mutex API.

Signed-off-by: Rahix <rahix@rahix.de>