wifi: mac80211: always initialize link_sta with sta

When we have multiple interfaces receiving the same frame,
such as a multicast frame, one interface might have a sta
and the other not. In this case, link_sta would be set but
not cleared again.

Always set link_sta, so we keep an invariant that link_sta
and sta are either both set or both not set.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>