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