ar71xx: fix TL-WR842N v2 switch port order

The TL-WR842N v2 has the following switch port layout with swconfig port 0
being connected to eth1:

            Power  LAN4    LAN3    LAN2    LAN1    WAN     USB
             (*) | [...] | [...] | [...] | [...] | [...] | [.]
    swconfig port:   1       4       3       2      n/a
    netdev:        eth1    eth1    eth1    eth1    eth0

Adjust the board defaults to match this model specific layout.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>