batman-adv: Return EINVAL on invalid gw_bandwidth change

Invalid speed settings by the user are currently acknowledged as correct
but not stored. Instead the return of the store operation of the file
"gw_bandwidth" should indicate that the given value is not acceptable.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>