[SCSI] mpt fusion: Usage of high priority request FIFO to send task management commands

Added support for sending the task management requests through High priority
request FIFO instead of Doorbell writes when firmware support High priority
FIFO.

signed-off-by: Sathya Prakash <sathya.prakash@lsi.com>
Acked-by: Eric Moore <Eric.Moore@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>