mt76: mt7921: introduce mt7921_wpdma_reinit_cond utility routine

Add mt7921_wpdma_reinit_cond to check dummy reg if driver needs to
reinitialized WPDMA after driver_own operation

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Co-developed-by: Leon Yen <leon.yen@mediatek.com>
Signed-off-by: Leon Yen <leon.yen@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>