openwrt: backport DFS fixes (#2077)

This backports two fixes related to operation on DFS-required
channels.

When a DFS-required channel was selected as the regular
(non-outdoor-mode) 5 GHz channel, hostapd would switch to a non-DFS
channel as OpenWrt did not pass a chanlist of allowed ACS channels.

When hostapd is given a single channel for the chanlist or there's no
available channel left (all allowed channels are in the no-occupancy
period), hostapd prints "no DFS channels left, waiting for NOP to
finish" to the syslog but never stopped transmitting on this channel,
still sending out beacon frames and allowing client data transfer.