mac80211: create channel list for fixed channel operation

mac80211: create channel list for fixed channel operation

Currently a device which has a DFS channel selected using the UCI
channel setting might switch to a non-DFS channel in case no chanlist is
provided (UCI setting "channels") when the radio detects a DFS event.

Automatically add a chanlist consisting of the configured channel when
the device does not operate in auto-channel mode and no chanlist set to
circumvent this issue.

Signed-off-by: David Bauer <mail@david-bauer.net>