[packages/openvpn] fix changing ipv6 settings with busybox's ifconfig

[packages/openvpn] fix changing ipv6 settings with busybox's ifconfig

The OpenVPN call to busybox's ifconfig in order to change interface
configurations is incompatible when used in conjunction with IPv6.
Busybox's ifconfig doesn't know about the 'inet6' parameter,
so just skip it.
The new call is still compatible to the usual ifconfig provided by
the net-tools package.

Thanks to Gert Doering and Joachim Schlipper!

git-svn-id: svn://svn.openwrt.org/openwrt/packages@33375 3c298f89-4303-0410-b956-a3cf2f4a3e73