ibmveth: Add function to enable live MAC address changes

Add a function that will enable changing the MAC address
of an ibmveth interface while it is still running.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>