kernel: mark kmod-usb-phy-nop as hidden to avoid building it where it is not needed

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40470 3c298f89-4303-0410-b956-a3cf2f4a3e73