Update the board MAC address configuration for the DIR-655A1.

The MAC addresses were totally wrong. They're like the DIR-625C1 - at
0x1ffe0004 and 0x1ffe0018. They're however stored as text strings.
The ath0 MAC address is also not set, even though the calibration
partition is valid.

So, pick the board address / first MAC as the ath0 MAC, and derive
arge0/arge1 from that. That way they're hopefully unique enough
for people with multiple devices.

Tested:

• DIR-655A1

TODO:

• Do the same for the DIR-625A1 and DIR-625C1.