scsi: sr: Use {get,put}_unaligned_be*() instead of open-coding these functions

This patch makes the sr code slightly easier to read.

Link: https://lore.kernel.org/r/20200427014844.12109-1-bvanassche@acm.org


Cc: Merlijn Wajer <merlijn@archive.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>