mt76: mt7921: fix mt7921_wfsys_reset sequence

WiFi subsytem reset should control MT_WFSYS_SW_RST_B and then poll the
same register until the bit WFSYS_SW_INIT_DONE bit is set.

Fixes: 0c1ce988 ("mt76: mt7921: add wifi reset support")
Reviewed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>