mt76: connac: fix GTK rekey offload failure on WPA mixed mode

Update the proper firmware programming sequence to fix GTK rekey
offload failure on WPA mixed mode.

In the mt76_connac_mcu_key_iter,
gtk_tlv->proto should be only set up on pairwise key
and gtk_tlk->group_cipher should be only set up on the group key.

Otherwise, those parameters required by firmware would be set
incorrectly to cause GTK rekey offload failure on WPA mixed mode
and then disconnection follows.

Fixes: b47e21e7 ("mt76: mt7615: add gtk rekey offload support")
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Leon Yen <Leon.Yen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>