We won't ever queue more commands than the host allows.  Instead of
letting drivers either reject or ignore this case handle it in
common code.  Note that various driver use internal constant or
variables that are assigned to both shost->can_queue and checked
in ->change_queue_depth - I did remove those checks as well.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NMike Christie <michaelc@cs.wisc.edu>
Reviewed-by: NHannes Reinecke <hare@suse.de>