scsi: mac_scsi: Refactor polling loop

Before the error handling can be revised, some preparation is needed.
Refactor the polling loop with a new function, macscsi_wait_for_drq().
This function will gain more call sites in the next patch.

Cc: stable@vger.kernel.org # 5.15+
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Link: https://lore.kernel.org/r/6a5ffabb4290c0d138c6d285fda8fa3902e926f0.1723001788.git.fthain@linux-m68k.org


Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>