drivers: flash: sam0: Resolve write issues in the first row

Previously, while writing close to the start of flash (i.e: in the first
row), the write was never committed - due to zero being used as the
"empty buffer" marker.

This patch also incorporates some performance improvements, as well as
observing any errors reported by `flash_sam0_commit()`.

Fixes #52309

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