wifi: ath9k_htc: Abort software beacon handling if disabled

wifi: ath9k_htc: Abort software beacon handling if disabled

A malicious USB device can send a WMI_SWBA_EVENTID event from an
ath9k_htc-managed device before beaconing has been enabled. This causes
a device-by-zero error in the driver, leading to either a crash or an
out of bounds read.

Prevent this by aborting the handling in ath9k_htc_swba() if beacons are
not enabled.

Reported-by: Robert Morris <rtm@csail.mit.edu>
Closes: https://lore.kernel.org/r/88967.1743099372@localhost
Fixes: 832f6a18fc2a ("ath9k_htc: Add beacon slots")
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://patch.msgid.link/20250402112217.58533-1-toke@toke.dk
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>