drivers: i2c: sam0: Resolve spurious trailing 0xff on reads

Previously, the I2C bus would have an extended idle period on write
operations, and a spurious trailing byte on read operations (after a
final NACK).

This patch relocates the issuance of the stop condition from the driving
thread's context, into the ISR.

Thanks to @benediktibk for discovery and initial patch.

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>