wifi: mt76: fix coverity uninit_use_in_call in mt76_connac2_reverse_frag0_hdr_trans()

The default case for frame_contorl is invalid. We should always
assign addr3 of this frame properly.

Coverity error message:
if (ieee80211_has_a4(hdr.frame_control))
(19) Event uninit_use_in_call:	Using uninitialized value "hdr".
Field "hdr.addr3" is uninitialized when calling "memcpy".
	memcpy(skb_push(skb, sizeof(hdr)), &hdr, sizeof(hdr));
else
	memcpy(skb_push(skb, sizeof(hdr) - 6), &hdr, sizeof(hdr) - 6);

Fixes: 0880d408 ("mt76: connac: move mt76_connac2_reverse_frag0_hdr_trans in mt76-connac module")
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>