mac80211: exclude AP_VLAN interfaces from tx power calculation

Their power value is initialized to zero. This patch fixes an issue
where the configured power drops to the minimum value when AP_VLAN
interfaces are created/removed.

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