omap3: zoom 2/3: Change debugboard serial port id

This is now changed to PLAT8250_DEV_PLATFORM (= 0), because
it's the only port that's going to be initialized in
Zoom 2/3 boards.

So, it doesn't make sense to keep the hardcoded 3 value anymore.
Signed-off-by: NSergio Aguirre <saaguirre@ti.com>

arch/arm/mach-omap2/board-zoom-debugboard.c

@@ -96,7 +96,7 @@ static struct plat_serial8250_port serial_platform_data[] = {
 
 static struct platform_device zoom_debugboard_serial_device = {
 	.name			= "serial8250",
-	.id			= 3,
+	.id			= PLAT8250_DEV_PLATFORM,
 	.dev			= {
 		.platform_data	= serial_platform_data,
 	},