ramips: fix IPv4-only corner case in 01_enable_packet_steering

ramips: fix IPv4-only corner case in 01_enable_packet_steering

The uci config section network.globals set up in /bin/config_generate
will only be created if /proc/sys/net/ipv6 exists.

Correspondingly, lacking IPv6 support, the command
uci set network.globals.packet_steering=1
will fail with "uci: Invalid argument" as the network.globals config
has not been set up.

Fix that by adding the setup there as well.

Fixes: dfd62e575c6c ("ramips: enable packet steering by default on mt7621")
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>