[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/arch/arm/mach-ixp4xx/gtwx5715-setup.c b/arch/arm/mach-ixp4xx/gtwx5715-setup.c
index 3fd92c5..29a6d02 100644
--- a/arch/arm/mach-ixp4xx/gtwx5715-setup.c
+++ b/arch/arm/mach-ixp4xx/gtwx5715-setup.c
@@ -93,7 +93,7 @@
 
 static struct platform_device gtwx5715_uart_device = {
 	.name		= "serial8250",
-	.id		= 0,
+	.id		= PLAT8250_DEV_PLATFORM,
 	.dev			= {
 		.platform_data	= gtwx5715_uart_platform_data,
 	},