scsi: mptscsih: Remove bogus interpretation of request->ioprio

Having an I/O priority does not mean we should send all requests as HEAD
OF QUEUE tags.

Reported-by: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>