lightnvm: handle submit_io failure

The device ->submit_io() callback might fail to submit I/O to device.
In that case, the nvm_submit_ppa function should not wait for
completion. Instead return the ->submit_io() error.

Reviewed by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>