usb: gadget: add dummy function for HTC libril_htc.so

HTC's libhtc_ril calls into the USB gadget, provide infrastructure to
add dummy functions so it won't crash. Add dummy function for
usb_function_switch to fix libhtc_ril.so crashing on screen-off. This
should enable libhtc_ril.so from RUU 3.25+ to work as well.