nvme: allow for size limitations from transport drivers

Some transport drivers may have a lower transfer size than
the controller. So allow the transport to set it in the
controller max_hw_sectors.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>