lightnvm: wrong return value and redundant free

The return value should be non-zero under error conditions.
Remove nvme_free(dev) to avoid free dev more than once.

Signed-off-by: Wenwei Tao <ww.tao0320@gmail.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>