usb: gadget: Relocate f_rndis

Relocate f_rndis driver to function/ directory and
fix following configfs + rndis function build failure:
----------
make[4]: *** No rule to make target `drivers/usb/gadget/function/f_rndis.o', needed by `drivers/usb/gadget/function/usb_f_rndis.o'.  Stop.
make[3]: *** [drivers/usb/gadget/function] Error 2
make[2]: *** [drivers/usb/gadget] Error 2
make[1]: *** [drivers/usb] Error 2
make: *** [drivers] Error 2
----------

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