ath9k: fix a scheduling while atomic bug in CSA handling

Commit "ath9k: prepare for multi-interface CSA support" added a call to
ieee80211_iterate_active_interfaces in atomic context (beacon tasklet),
which is crashing.
Use ieee80211_iterate_active_interfaces_atomic instead.

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