openvswitch: Use eth_mac_addr() instead of duplicating it

bonus: if we ever are to use IFF_LIVE_ADDR_CHANGE for
anything further than to check availability in eth_mac_addr(),
Open vSwitch will be ready for that.

Signed-off-by: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>