scsi: st: Use get_unaligned_be24() and sign_extend32()

Use these functions instead of open-coding them.

Link: https://lore.kernel.org/r/20200313203102.16613-5-bvanassche@acm.org


Cc: Kai Makisara <Kai.Makisara@kolumbus.fi>
Cc: James E.J. Bottomley <jejb@linux.ibm.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>