mac80211: fix idle handling in monitor mode

When the driver does not want a monitor mode VIF, no channel context is
allocated for it. This causes ieee80211_recalc_idle to put the hardware
into idle mode if only a monitor mode is active, breaking injection.

Fix this by checking local->monitors in addition to active channel
contexts.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>