[SCSI] qla2xxx: Set host can_queue value based on available resources.

Tell the mid-layer that number of commands we can queue is the available
resources we have minus a small amount for internal commands.

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>