ar71xx: fix MAC addresses for Archer C5 v1, C7 v1/v2, WDR4900 v2

ar71xx: fix MAC addresses for Archer C5 v1, C7 v1/v2, WDR4900 v2

As discussed in 1d18a14a90c7 ("ath79: really fix TP-Link Archer C7
v2 MAC address"), stock firmware MAC address assignment is
actually as follows:

wlan0 (5GHz) : -1
wlan1 (2.4GHz) : 0
eth1 (LAN) : 0
eth0 (WAN) : 1

This has never been fixed for ar71xx, so let's do it now.
Note that with WDR4900 v2 even both wlan0 and wlan1 where assigned
to basemac-1 before ...

Fixes: FS#408

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit a021268032fb64afdce7bafa91c7c7bf44fa6ff2)