drivers: i2c: mchp_mss: Rework

Use a mutex to protect the I2C bus from concurrent access.  Replace busy
waiting with a binary semaphore.  Process the messages in the interrupt
handler state machine to generate a single transaction.  In the
interrupt handler, process the states until nothing more is to do.  This
prevents superfluous interrupt entry/exit pairs.  Return error
conditions to the caller.  Check the messages for consistency and return
an error for unsupported message lists.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>