Update hw_mode when CSA finishes

Update hw_mode when CSA finishes

The driver might decide to change the operating band on its own, e.g.,
when trying to use a single channel in AP + AP case. A CSA event will be
notified to hostapd to update the channel/frequency, but hw_mode did not
get updated accordingly.

This may cause hostapd interface restarting to fail, e.g., with control
interface DISABLE / ENABLE commands at configured_fixed_chan_to_freq(),
because of the mismatch between conf->channel and conf->hw_mode.

Update hw_mode right after channel change to fix this.

Signed-off-by: ngong <quic_ngong@quicinc.com>