mac80211: fix encryption key selection for 802.3 xmit

When using WEP, the default unicast key needs to be selected, instead of
the STA PTK.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20201218184718.93650-4-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>