macvlan: add support for ethtool get settings

If macvlan's are used, it is useful to propgate speed and other settings
from underlying device up for application usage.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>