wifi: mt76: mt792x: add PSE handling barrier for the large MCU cmd

wifi: mt76: mt792x: add PSE handling barrier for the large MCU cmd

Add a dummy register read in mt76_connac_mcu_rate_txpower_band() to act as
a PSE barrier. This would release PSE pages and prevents buffer underflow
issues when handling MCU commands with larger payloads without the response
in mt76_connac_mcu_set_rate_txpower().

This is a prerequisite patch before enabling MT7902 PCIe and SDIO support.

Co-developed-by: Xiong Huang <xiong.huang@mediatek.com>
Signed-off-by: Xiong Huang <xiong.huang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Link: https://patch.msgid.link/20260219004007.19733-6-sean.wang@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>