ar71xx: fix bad RX sensitivity on newer TL-WDR4300 boards

On recent TL-WDR4300 boards the external LNAs of the 2.4GHz
interface are connected to GPIO lines. Because these GPIO
lines are disabled by default, the RX sensitivity of the
device is quite bad.

Setup the GPIOs of the external LNAs to fix the issue.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39392 3c298f89-4303-0410-b956-a3cf2f4a3e73