ip6_gre: remove the incorrect mtu limit for ipgre tap

The same fix as the patch "ip_gre: remove the incorrect mtu limit for
ipgre tap" is also needed for ip6_gre.

Fixes: 61e84623 ("net: centralize net_device min/max MTU checking")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>