RaspberryPi/RPi3: Update USB network driver path
When OptionRomPkg moved from EDK2 to EDK2-platforms, the relative
search path within the repo changed.
Update UsbNetworking/Ax88772b/Ax88772b.inf references to match.
Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>