[SCSI] stex: cancel unused field in struct req_msg

The payload_sz field in struct req_msg is not big enough to indicate
the size of req_msg, as its type is u8.
It is confirmed that this field is not used by firmware, so cancel
it here.

Signed-off-by: Ed Lin <ed.lin@promise.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>