ARM: rpi: only set usbethaddr on relevant systems

Model A and CM RPis don't have an on-board USB Ethernet device. Hence,
there's no point setting $usbethaddr based on the device fuses. Use the
model detection code to gate this. Note that the fuses are actually
programmed even on those devices though.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>