mpt2sas: Remove uses of serial_number

The mpt2sas_scsih_issue_tm() function does not use the 'serial_number'
argument passed to it.  Removing it removes the last vestiges of the
scsi_cmnd's serial_number field from this driver.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Reviewed-by: Nicholas Bellinger <nab@linux-iscsi.org>
Reviewed-by: Praveen Krishnamoorthy <Praveen.krishnamoorthy@lsi.com>
Acked-by: Sreekanth Reddy <Sreekanth.reddy@lsi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>