cfg80211: use the correct macro to check for active monitor support

Use MONITOR_FLAG_ACTIVE, which is a flag mask, instead of
NL80211_MNTR_FLAG_ACTIVE, which is a flag index, when checking if the
hardware supports active monitoring.

Cc: stable@vger.kernel.org
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>