hostapd: Add support for DFS channels in CHAN_SWITCH

hostapd: Add support for DFS channels in CHAN_SWITCH

Enable support for DFS channels in the CHAN_SWITCH command. Perform CAC
instead of CSA if DFS channel is selected. Then restart normal AP
operations.

Note that the current implementation provides a simplified approach. It
does not check if the selected DFS channel block is already in the
HOSTAPD_CHAN_DFS_AVAILABLE state. CAC procedure is restarted anyway.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>