mt76: mt76x02: only override control->sta on sw-encrypted tx

control->sta is set to NULL early when encryption is turned on for the
station and info->control.hw_key is not set.
This code is missing a check for the 802.11 header protected flag, otherwise
it resets the station for other frames, e.g. client probing frames.

Signed-off-by: Felix Fietkau <nbd@nbd.name>