[SERIAL] Use an enum for serial8250 platform device IDs

Rather than hard-coding the platform device IDs, enumerate them.
We don't particularly care about the actual ID we get, just as
long as they're unique.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/drivers/serial/8250_mca.c b/drivers/serial/8250_mca.c
index f0c40d6..ac20525 100644
--- a/drivers/serial/8250_mca.c
+++ b/drivers/serial/8250_mca.c
@@ -44,7 +44,7 @@
 
 static struct platform_device mca_device = {
 	.name			= "serial8250",
-	.id			= 5,
+	.id			= PLAT8250_DEV_MCA,
 	.dev			= {
 		.platform_data	= mca_data,
 	},