mac80211: exclude AP_VLAN interfaces from tx power calculation

commit 764152ff66f4a8be1f9d7981e542ffdaa5bd7aff upstream.

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.

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