ath9k: fix beacon slot processing in ad-hoc mode

The recent cleanups in the beacon code fixed SWBA backoff calculation,
however it did not remove a line of code that worked around the issues
from the earlier version of the code.
After the cleanup, the initial TSF based slot calculation now always
returns 0 instead of ATH_BCBUF-1, so the previous hack that reversed the
slot order needs to be removed, as ad-hoc mode does not use staggered
beacons.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>