lightnvm: prematurely activate nvm_dev

We register with nvm_devices when there registration can still fail.
Move the final registration at the end of the nvm_register function
to make sure we are fully registered when added to the nvm_devices list.

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