mt76: mt7663s: use NULL instead of 0 in sdio code

Fix the following sparse warnings in mt7663s driver:

drivers/net/wireless/mediatek/mt76/mt7615/sdio_mcu.c:78:62: warning:
Using plain integer as NULL pointer
drivers/net/wireless/mediatek/mt76/mt7615/sdio_mcu.c:110:62: warning:
Using plain integer as NULL pointer
drivers/net/wireless/mediatek/mt76/mt7615/sdio_txrx.c:229:64: warning:
Using plain integer as NULL pointer
drivers/net/wireless/mediatek/mt76/mt7615/sdio_txrx.c:263:64: warning:
Using plain integer as NULL pointer

Fixes: a66cbdd6 ("mt76: mt7615: introduce mt7663s support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>