wifi: cfg80211: optionally support monitor on disabled channels

wifi: cfg80211: optionally support monitor on disabled channels

If the hardware supports a disabled channel, it may in
some cases be possible to use monitor mode (without any
transmit) on it when it's otherwise disabled. Add a new
channel flag IEEE80211_CHAN_CAN_MONITOR that makes it
possible for a driver to indicate such a thing.

Make it per channel so drivers could have a choice with
it, perhaps it's only possible on some channels, perhaps
some channels are not supported at all, but still there
and marked disabled.

In _nl80211_parse_chandef() simplify the code and check
only for an unknown channel, _cfg80211_chandef_usable()
will later check for IEEE80211_CHAN_DISABLED anyway.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240206164849.87fad3a21a09.I9116b2fdc2e2c9fd59a9273a64db7fcb41fc0328@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>