lightnvm: remove chnl_offset in nvme_nvm_identity

The identity structure is initialized to zero in the beginning of
the nvme_nvm_identity function. The chnl_offset is separately set to
zero. Since both the variable and assignment is never changed, remove
them.

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