mt76: connac: fw_own rely on all packet memory all being free

If the device is MMIO-based, we must ensure all TxD/TxP on the host
memory all being consumed by the device prior to safely switching to
fw_own state.

Fixes: ec7bd7b4 ("mt76: connac: check wake refcount in mcu_fw_pmctrl")
Reviewed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>