lightnvm: use target nvm on target-specific ops.

On target-specific operations pass on nvm_tgt_dev instead of the generic
nvm device.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>