usb: gadget: configfs: handle gadget reset request for android

There is this new mandatory UDC->reset API in v3.18+ kernels,
commit ef979a26 "usb: gadget: add reset API at usb_gadget_driver".

Let android_disconnect handle that for Android, similar to
how composite_disconnect is handling the generic ConfigFS
gadget reset request.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>