hpsa: check for a null phys_disk pointer in ioaccel2 path

An oops can occur when submitting ioaccel2 commands when the phys_disk
pointer is NULL in hpsa_scsi_ioaccel_raid_map.  Happens when there are
configuration changes during I/O operations.

If the phys_disk pointer is NULL, send the command down the RAID path.

Reviewed-by: Mahesh Rajashekhara <mahesh.rajashekhara@microsemi.com>
Reviewed-by: Justin Lindley <justin.lindley@microsemi.com>
Reviewed-by: Scott Teel <scott.teel@microsemi.com>
Reviewed-by: Kevin Barnett <kevin.barnett@microsemi.com>
Reviewed-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>