wpa_supplicant: correct legal info

wpa_supplicant is licensed under BSD-3c only and not dual licensed.

Quoting from COPYING file
 This software was previously distributed under BSD/GPL v2 dual license
 terms that allowed either of those license alternatives to be
 selected. As of February 11, 2012, the project has chosen to use only
 the BSD license option for future distribution. As such, the GPL v2
 license option is no longer used.

Reviewed-by: Abhimanyu Vishwakarma <Abhimanyu.Vishwakarma@imgtec.com>
Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>