Bluetooth: btmtksdio: use NULL instead of zero

Fix sparse warnings:

drivers/bluetooth/btmtksdio.c:499:57: warning: Using plain integer as NULL pointer
drivers/bluetooth/btmtksdio.c:533:57: warning: Using plain integer as NULL pointer
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/btmtksdio.c

@@ -496,7 +496,7 @@ static void btmtksdio_interrupt(struct sdio_func *func)
 	sdio_claim_host(bdev->func);
 
 	/* Disable interrupt */
-	sdio_writel(func, C_INT_EN_CLR, MTK_REG_CHLPCR, 0);
+	sdio_writel(func, C_INT_EN_CLR, MTK_REG_CHLPCR, NULL);
 
 	int_status = sdio_readl(func, MTK_REG_CHISR, NULL);
 
@@ -530,7 +530,7 @@ static void btmtksdio_interrupt(struct sdio_func *func)
 	}
 
 	/* Enable interrupt */
-	sdio_writel(func, C_INT_EN_SET, MTK_REG_CHLPCR, 0);
+	sdio_writel(func, C_INT_EN_SET, MTK_REG_CHLPCR, NULL);
 
 	pm_runtime_mark_last_busy(bdev->dev);
 	pm_runtime_put_autosuspend(bdev->dev);