mg_disk: fix enum REQ_OP_ kbuild error

Because we define WRITE/READ as REQ_OPs, we cannot do
switch (rq_data_dir(request))
case READ
....
case WRITE
...

without getting warnings about handling other REQ_OPs.

This just has mq_disk do a if/else like it does in other
places.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>