ahci: enable full queue depth of 32

This changes the AHCI queue depth from 31 to 32, as libata now
fully supports it. Now regular IO requests can utilize the full
tag space of SATA, not just 31. For IOPS constrained workloads,
this can result in a ~3% bump in performance.

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