nvme: set max_hw_sectors unconditionally

All transports set a max_hw_sectors value in the nvme_ctrl, so make
the code using it unconditional and clean it up using a little helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>