DTR GPIO isn't actually needed and triggers boot warning.
TX pin was off by one (GPIO 19 instead of GPIO 18).
Reported-by: @tophirsch
Fixes: d1130ad265 ("ath79: add support for Teltonika RUT955")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 53a7d5d614)