ar71xx: Fix WD My Net N750 weak signal issue

ar71xx: Fix WD My Net N750 weak signal issue

The LNAs need to be enabled by setting their respective GPIO to high even
though the original firmware's setting sets them to low on initialization.

Obviously the LNAs are then later initialized by the driver on the OEM
firmware. Without this fix the device is mostly "deaf".

Signed-off-by: Felix Kaechele <heffer@fedoraproject.org>
Patchwork: http://patchwork.openwrt.org/patch/4688/
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
SVN-Revision: 39213