lightnvm: use kobj_to_dev()

This fixs coccicheck warning:

drivers/nvme//host/lightnvm.c:1243:60-61: WARNING opportunity for
kobj_to_dev()

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Matias Bjørling <matias.bjorling@wdc.com>
Link: https://lore.kernel.org/r/20210413105257.159260-2-matias.bjorling@wdc.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>