nvdimm-btt: Use the enum req_op type

Improve static type checking by using the enum req_op type where
appropriate.

Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-20-bvanassche@acm.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>