serial: zoom2: Remove zoom2 serial prototypes from serial.h

Remove the prototypes for zoom2_serial_deviceN from serial.h . This
can't be done right away, as they are referenced from the zoom2
config file. Therefore, adjust the code so the config file only
specifies number of the port. Then, replace the simple return in
default_serial_console() with a switch across possible values, which
returns the zoom2_serial_deviceN . With such adjustment in place,
the exported prototypes in serial.h can be safely removed.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
diff --git a/include/configs/omap3_zoom2.h b/include/configs/omap3_zoom2.h
index 4447dff..a7cc5fc 100644
--- a/include/configs/omap3_zoom2.h
+++ b/include/configs/omap3_zoom2.h
@@ -81,7 +81,7 @@
  * 0 - 1 : first  USB with respect to the left edge of the debug board
  * 2 - 3 : second USB with respect to the left edge of the debug board
  */
-#define ZOOM2_DEFAULT_SERIAL_DEVICE	(&zoom2_serial_device0)
+#define ZOOM2_DEFAULT_SERIAL_DEVICE	0
 
 #define V_NS16550_CLK			(1843200)	/* 1.8432 Mhz */