usb: gadget: net2280: fix memory leak on probe error handling paths

Driver does not release memory for device on error handling paths in
net2280_probe() when gadget_release() is not registered yet.

The patch fixes the bug like in other similar drivers.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Evgeny Novikov <novikov@ispras.ru>
Signed-off-by: Felipe Balbi <balbi@kernel.org>