ramips: various Netgear R6120 fixes

ramips: various Netgear R6120 fixes

The R6120 has no 5GHz WLAN LED, the assigned GPIO in fact controls
the WAN LED.

Renames the LED accordingly in the device-tree.
Removes the 5GHz WLAN LED trigger.
Adds the correct WAN port LED trigger.

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Currently, the MAC address for the Netgear R6120 is read from the NVRAM
partition. The offset for the MAC address however is not consistent
across devices or firmware versions.

Switch to using the factory partition like all other Netgear devices do.

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The LAN ports of the R6120 are labled in reverse on the casing.

Adjust LuCI switchport numbering accordingly.

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The WiFi eeprom offsets for the R6120 are currently wrong (5GHz offset
is bigger than the partition itself).

Fixes poor performance on 2.4 and 5 GHz.

Signed-off-by: David Bauer <mail@david-bauer.net>