megaraid: fix null pointer check in megasas_detach_one().

The pd_seq_sync pointer can't be NULL, we have to check its entries
instead.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>