[SCSI] qla2xxx: Determine the number of outstanding commands based on available resources.

Base the number of outstanding requests the driver will keep track of on the
available resources instead of being hard-coded.

Signed-off-by: Chad Dupuis <chad.dupuis@qlogic.com>
Signed-off-by: Saurav Kashyap <saurav.kashyap@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>