drivers: sensor: bmm350: fix handling interrupt race condition

There is a race condition where an interrupt can fire before the
drdy_handler is registered. The drdy_handler will tradionaly
callback the sample get clearing the interrupt... but if it's not
configured and NULL, the interrupt will stay forever latched. Read
the interrupt status to clear the interrupt flag allowing it to
trigger again.

Also, move the serial api function helpers in to the header
allowing them to be used from other c files.

Signed-off-by: Ryan McClelland <ryanmcclelland@meta.com>