ramips: add label MAC address for Edimax RG21S

ramips: add label MAC address for Edimax RG21S

The Edimax RG21S has a label which bears two MAC addresses:
2.4 GHz (n) and 5 GHz (n+1)

The complete MAC address setup is as follows:
2.4 GHz  *:83  factory 0x4, u-boot-env wlanaddr
5 GHz    *:84  factory 0x8004
LAN      *:83  u-boot-env ethaddr
WAN      *:85  u-boot-env wanaddr

Since 2.4 GHz is the first address on the label and the same
as used for ethernet, take this one for label MAC address.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>