ath9k: fix DFS detector synchronization

some userspace programs (e.g. hostapd) need to set the regulatory domain
before selecting the operating channel. Synchronize DFS detector regardless of
the value of ah->curchan, to avoid situations where wireless scan can't be done
on some 5GHz sub-bands, because dfs_region is constantly UNSET.

Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>