nvme: simplify the max_discard_segments calculation

Just stash away the DMRL value in the nvme_ctrl struture, and leave
all interpretation to nvme_config_discard, where we know DSM is
supported by the time we're configuring the number of segments.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>