mpt3sas: Make use of additional HighPriority credit message frames for sending SCSI IO's

Driver assumes HighPriority credit as part of Global credit. But,
Firmware treats HighPriority credit value and global cedits as two
different values. Changed host queue algorithm to treat global credits
and highPriority credits as two different values.

Signed-off-by: Suganath prabu Subramani <suganath-prabu.subramani@avagotech.com>
Signed-off-by: Chaitra P B <chaitra.basappa@avagotech.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>