ramips: fix WiFi MAC addresses for D-Link DIR-810L

ramips: fix WiFi MAC addresses for D-Link DIR-810L

So far, WiFi MAC addresses for this device have been set up from
caldata. However, this returns values which do not look like MAC
addresses. They also do not match stock firmware:

wlan0 (5.0): 00:11:22:00:17:D0 from 0x8004
wlan1 (2.4): 00:11:22:00:17:CD from 0x4 (and 0x2e)

It looks like the only valid MAC address on this device is at 0x28.

So, this patch changes setup to calculate addresses based on the
value at 0x28:

lan: *:0A (flash, label)
wan: *:0B (flash + 1)
wifi2: *:0A (flash)
wifi5: *:0C (flash + 2)

Thanks to Roger Pueyo Centelles <roger.pueyo@guifi.net> for
investigating this on his devices.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit d1072096f49823eb39357f9555d7854a9c91bcfb)