lightnvm: reduce number of nvm_id groups to one

The number of configuration groups has been limited to one in current
code, even if there is support for up to four. With the introduction
of the open-channel SSD 1.3 specification, only a single
group is exposed onwards. Reflect this in the nvm_id structure.

Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@fb.com>