ip6gre: Allow live link address change

The ip6 GRE tap device should not be forced to down state to change
the mac address and should allow live address change for tap device
similar to ipv4 gre.

Signed-off-by: Shweta Choudaha <schoudah@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>