nvme: lightnvm: clean up a data type

"nlb_pr_rq" can't be more than u32 because "len" is a u32.  Later we
truncate it to u32 anyway when we calculate min_t().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>