mediatek: fix IPv4-only corner case and commit in 99-net-ps

mediatek: fix IPv4-only corner case and commit in 99-net-ps

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.

While at it, limit the uci commit to the network config file.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 636b62e8e232951154ac4ccd7972fda3f8de0379)