ibm_newemac Use status property for unused/unwired EMACs

Convert ibm_newemac to use the of_device_is_available function when checking
for unused/unwired EMACs.  We leave the current check for an "unused" property
to maintain backwards compatibility for older device trees.  Newer device
trees should simply use the standard "status" property in the EMAC node.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>