Add helper to convert message byte into a host byte code.

Link: https://lore.kernel.org/r/20210427083046.31620-18-hare@suse.deReviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>