dm: serial: Adjust serial_getconfig() to use proper API

All driver-model functions should have a device as the first parameter.
Update this function accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
diff --git a/arch/x86/lib/acpi_table.c b/arch/x86/lib/acpi_table.c
index 79bc200..bfcf2ad 100644
--- a/arch/x86/lib/acpi_table.c
+++ b/arch/x86/lib/acpi_table.c
@@ -342,6 +342,7 @@
 	struct acpi_table_header *header = &(spcr->header);
 	struct serial_device_info serial_info = {0};
 	ulong serial_address, serial_offset;
+	struct udevice *dev;
 	uint serial_config;
 	uint serial_width;
 	int access_size;
@@ -431,7 +432,9 @@
 		break;
 	}
 
-	ret = serial_getconfig(&serial_config);
+	ret = uclass_first_device_err(UCLASS_SERIAL, &dev);
+	if (!ret)
+		ret = serial_getconfig(dev, &serial_config);
 	if (ret)
 		serial_config = SERIAL_DEFAULT_CONFIG;