lightnvm: flatten nvm_id_group into nvm_id

There are no groups in the 2.0 specification, make sure that the
nvm_id structure is flattened before 2.0 data structures are added.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>