boards: Fixing rak4631 lora RX and TX pins

Taking a look at the schematic shows that
tx-enable-gpios and dio2-tx-enable were directly connected.
This had the effect tx-enable-gpios was trying to pull the line down
while SX1262 is pulling the same line high. This increased
the power consumption.
Now only the SX1262 sets the antenna to tx via dio2-tx-enable.
Setting the antenna to rx is still done via the rx-enable-gpios.
The antenna defaults to rx if not in use.

Signed-off-by: Carlo Weidinger <carlo.weidinger@protonmail.com>