classmate-laptop: Fix sparse warning (0 as NULL)

Fix the following sparse warning:

classmate-laptop.c:523:61: warning: Using plain integer as NULL pointer
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>

drivers/platform/x86/classmate-laptop.c

@@ -520,7 +520,7 @@ static acpi_status cmpc_get_accel(acpi_handle handle,
 {
 	union acpi_object param[2];
 	struct acpi_object_list input;
-	struct acpi_buffer output = { ACPI_ALLOCATE_BUFFER, 0 };
+	struct acpi_buffer output = { ACPI_ALLOCATE_BUFFER, NULL };
 	unsigned char *locs;
 	acpi_status status;