scsi: fas216: Introduce struct fas216_cmd_priv

Introduce a structure with driver-private data per SCSI command. This data
structure will be used by the next patch to store a function pointer.

Link: https://lore.kernel.org/r/20211007202923.2174984-33-bvanassche@acm.org


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