ataflop: potential out of bounds in do_format()

The function uses "type" as an array index:

	q = unit[drive].disk[type]->queue;

Unfortunately the bounds check on "type" isn't done until later in the
function.  Fix this by moving the bounds check to the start.

Fixes: bf9c0538

 ("ataflop: use a separate gendisk for each media format")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>