mt76: fix dfs state issue with 160 MHz channels

When operating on a mix of DFS and non-DFS channels, the driver only checks
the CAC status of the control channel. This causes beacons/tx to fail if the
control channel is on a non-DFS channel.
Fix this by calling cfg80211_reg_can_beacon to determine the DFS status of
all affected channels

Signed-off-by: Felix Fietkau <nbd@nbd.name>