sata_nv: set host can_queue count appropriately

libata limits the max limit for drivers to 31 anyway. We'll soon
allow drivers to actually go to QD=32, but that might require some
driver modifications. Before we do that, ensure that sata_nv limits
the depth to 31.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Tejun Heo <tj@kernel.org>