wifi: mt76: mt7921e: improve reliability of dma reset

The hardware team has advised the driver that it is necessary to first put
WFDMA into an idle state before resetting the WFDMA. Otherwise, the WFDMA
may enter an unknown state where it cannot be polled with the right state
successfully. To ensure that the DMA can work properly while a stressful
cold reboot test was being made, we have reordered the programming sequence
in the driver based on the hardware team's guidance.

The patch would modify the WFDMA disabling flow from

"DMA reset -> disabling DMASHDL -> disabling WFDMA -> polling and waiting
until DMA idle" to "disabling WFDMA -> polling and waiting for DMA idle ->
disabling DMASHDL -> DMA reset.

Where he polling and waiting until WFDMA is idle is coordinated with the
operation of disabling WFDMA. Even while WFDMA is being disabled, it can
still handle Tx/Rx requests. The additional polling allows sufficient time
for WFDMA to process the last T/Rx request. When the idle state of WFDMA is
reached, it is a reliable indication that DMASHDL is also idle to ensure it
is safe to disable it and perform the DMA reset.

Fixes: 0a1059d0

 ("mt76: mt7921: move mt7921_dma_reset in dma.c")
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Co-developed-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Co-developed-by: Wang Zhao <wang.zhao@mediatek.com>
Signed-off-by: Wang Zhao <wang.zhao@mediatek.com>
Signed-off-by: Quan Zhou <quan.zhou@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>