batman-adv: ELP - use wifi tx bitrate as fallback throughput

Some wifi drivers (e.g. ath10k) provide per-station rx/tx values but no
estimated throughput. Setting a better estimate than the default 1 MBit
makes these devices work well with B.A.T.M.A.N. V.

Signed-off-by: René Treffer <treffer@measite.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>