scsi: target: cxgbit: Fix cxgbit_fw4_ack()

Use the pointer 'p' after having tested that pointer instead of before.

Fixes: 5cadafb2

 ("target/cxgbit: Fix endianness annotations")
Cc: Varun Prakash <varun@chelsio.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191023202150.22173-1-bvanassche@acm.org
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>