luci-mod-network: migrate config to use "ports" option

LuCI supports only the newer method of specifying bridge ports using the
"ports" option. Offer users migration so they can configure their
network.

Example:

1. Before
config device
        option name 'br-lan'
        option type 'bridge'
        list ifname 'lan1'
        list ifname 'lan2'
        list ifname 'lan3'
        list ifname 'lan4'

2. After
config device
        option name 'br-lan'
        option type 'bridge'
        list ports 'lan1'
        list ports 'lan2'
        list ports 'lan3'
        list ports 'lan4'

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>