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

drivers/nvdimm/btt.c

@@ -1422,7 +1422,7 @@ static int btt_write_pg(struct btt *btt, struct bio_integrity_payload *bip,
 
 static int btt_do_bvec(struct btt *btt, struct bio_integrity_payload *bip,
 			struct page *page, unsigned int len, unsigned int off,
-			unsigned int op, sector_t sector)
+			enum req_op op, sector_t sector)
 {
 	int ret;