megaraid_sas: reserve commands for IOCTLs and internal DCMDs

megaraid_sas: reserve commands for IOCTLs and internal DCMDs

1) For fusion adapters, limit reserved frames for non SCSI commands to 8
   (3 for parallel IOCTLs + 5 for driver's internal DCMDs).
   Earlier reserved commands for non SCSI IO frames was set to 32, so with
   this implementation, increased per controller "can_queue".
   Behavior of MFI controllers will remain unchanged.

2) Optimize the code related to per controller's 'can_queue' setting.

Signed-off-by: Sumit Saxena <sumit.saxena@avagotech.com>
Signed-off-by: Chaitra Basappa <chaitra.basappa@avagotech.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>