bsp/ar71xx: use ath10k-ct for archer c7

This enables ad-hoc mode for 5 GHz wireless.

Fixes #2

Signed-off-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Christian Dresel <fff@chrisi01.de>
Reviewed-by: Steffen Pankratz <kratz00@gmx.de>