hostapd: Handle running out of DFS channels

hostapd: Handle running out of DFS channels

In scenarios where only DFS channels are available (e.g., outdoor,
special country codes), hostapd must be able to handle situations
where all are unavailable.

The two possibilities to get there are
1) while operating on the last available DFS channel a radar is
   detected
2) hostapd is started while all channels are unavailable

In both cases, hostapd instead of terminating should better
wait for the NOPs to pass and re-try operation after the CAC.

This patch provides that feature by using the condition
(iface->state == HAPD_IFACE_DFS && !iface->cac_started)
as NOP mode signature to retry operation from within
hostapd_dfs_nop_finished().

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>