batman-adv: Add function to convert string to batadv throughput

The code to convert the throughput information from a string to the
batman-adv internal (100Kibit/s) representation is duplicated in
batadv_parse_gw_bandwidth. Move this functionality to its own function
batadv_parse_throughput to reduce the code complexity.

Signed-off-by: Sven Eckelmann <sven@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>