ath9k: remove VEOL support for ad-hoc

With VEOL, Beacon transmission in ad-hoc does not currently work.
I believe for larger ad-hoc networks, VEOL is too unreliable, as
it can get beacon transmissions stuck during synchronization.
Use SWBA based beacon trasmission similar to AP mode instead.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Benoit Papillault <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>