sb/intel/lynxpoint: Ensure that dev->chip_info is not null

Use either a regular null check or `config_of` to avoid bugs.

Change-Id: I36a01b898c3e62423f27c2940b5f875b73e36950
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46665
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/southbridge/intel/lynxpoint/serialio.c

@@ -131,7 +131,7 @@ static void serialio_init_once(int acpi_mode)
 
 static void serialio_init(struct device *dev)
 {
-	struct southbridge_intel_lynxpoint_config *config = dev->chip_info;
+	struct southbridge_intel_lynxpoint_config *config = config_of(dev);
 	struct resource *bar0, *bar1;
 	int sio_index = -1;