[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/coyote-setup.c b/arch/arm/mach-ixp4xx/coyote-setup.c
index 8b2f253..050c927 100644
--- a/arch/arm/mach-ixp4xx/coyote-setup.c
+++ b/arch/arm/mach-ixp4xx/coyote-setup.c
@@ -66,7 +66,7 @@
 
 static struct platform_device coyote_uart = {
 	.name		= "serial8250",
-	.id		= 0,
+	.id		= PLAT8250_DEV_PLATFORM,
 	.dev			= {
 		.platform_data	= coyote_uart_data,
 	},