batman-adv: Change batman_adv.h license to MIT

The ISC license is considered as not recommended in "Linux kernel licensing
rules". It should only be used for existing code or for importing code from
a different project with that license.

But the kernel still has the similar sounding MIT/Expat license under the
preferred licenses. Switching to this license for this relatively new file
should therefore allow batman-adv to better follow the new licensing rules.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Matthias Schiffer <mschiffer@universe-factory.net>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>