This adds default tx and rx antennas, so that LuCI shows automatic as the default radio option for the antennas.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

Backport of r42085

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42350 3c298f89-4303-0410-b956-a3cf2f4a3e73