null_blk: fix type mismatch null_handle_cmd()

In null_handle_cmd() when device is configured as zoned, variable op is
decalred as an int, where it is used to hold values of type
REQ_OP_XXX which is of type enum req_opf. Change the type from
int to enum req_opf.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>