scsi: mpt3sas: Signedness bug in _base_get_diag_triggers()

The "trigger_flags" variable needs to be signed for the error checking to
work.

Link: https://lore.kernel.org/r/X9DZH37bYPHwSQRP@mwanda


Fixes: aec93e8e ("scsi: mpt3sas: Add persistent trigger pages support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>