NVMe: silence warning about unused 'dev'

Depending on options, we might not be using dev in nvme_cancel_io():

drivers/nvme/host/pci.c: In function ‘nvme_cancel_io’:
drivers/nvme/host/pci.c:970:19: warning: unused variable ‘dev’ [-Wunused-variable]
  struct nvme_dev *dev = data;
                   ^

So get rid of it, and just cast for the dev_dbg_ratelimited() call.

Fixes: 82b4552b ("nvme: Use blk-mq helper for IO termination")
Signed-off-by: Jens Axboe <axboe@fb.com>