mac80211: fix smps mode check for AP_VLAN

In ieee80211_sta_ps_deliver_wakeup, sdata->smps_mode is checked. This is
initialized only for the base AP interface, not the individual VLANs.

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