ar71xx: refactor TL-WAx50RE setup code

This way it is a bit easier to add other TP-LINK devices based on the
Atheros AP123 reference board with one LAN port.

Based-on: http://patchwork.openwrt.org/patch/4506/


Signed-off-by: Martijn Zilverschoon <thefriedzombie@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39071