ibm_newemac: Skip EMACs that are marked unused by the firmware

Depending on how the 44x processors are wired, some EMAC cells
might not be useable (and not connected to a PHY). However, some
device-trees may choose to still expose them (since their registers
are present in the MMIO space) but with an "unused" property in them.

Signed-off-by: Hugh Blemings <hugh@blemings.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>