lightnvm: pblk: remove unnecessary indirection

Call nvm_submit_io directly and remove an unnecessary indirection on the
read path.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>