mt76: mt7921: fix warning Using plain integer as NULL pointer

Changed "0 to NULL" to avoid below sparse warnings
.../mt76/mt7921/acpi_sar.c:64:68: sparse: Using plain integer as NULL pointer

Fixes: ab79a7d7a96f ("mt76: mt7921: introduce ACPI SAR support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>

mt7921/acpi_sar.c

@@ -61,7 +61,7 @@ mt7921_acpi_read(struct mt7921_dev *dev, u8 *method, u8 **tbl, u32 *len)
 static int
 mt7921_asar_acpi_read_mtcl(struct mt7921_dev *dev, u8 **table, u8 *version)
 {
-	*version = (mt7921_acpi_read(dev, MT7921_ACPI_MTCL, table, 0) < 0)
+	*version = (mt7921_acpi_read(dev, MT7921_ACPI_MTCL, table, NULL) < 0)
 		   ? 1 : 2;
 	return 0;
 }