usb: gadget: dummy: fix infinite loop because of missing loop decrement

The while loop never terminates because the loop counter i is never
decremented. Fix this by decrementing i.

Detected by CoverityScan, CID#751073 ("Infinite Loop")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>