common: Move serial_printf() to the serial header

Move this function header to serial.h since this function is clearly
related to serial. The function itself stays in console.c since we don't
have a single serial file. DM and non-DM each has a separate file so we
would have to either create a new common serial file, or repeat the
function in both serial.c and serial-uclass.c, neither of which seem
worthwhile.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/usb/gadget/ep0.c b/drivers/usb/gadget/ep0.c
index a36d9ec..6fabee2 100644
--- a/drivers/usb/gadget/ep0.c
+++ b/drivers/usb/gadget/ep0.c
@@ -37,6 +37,7 @@
  */
 
 #include <common.h>
+#include <serial.h>
 #include <usbdevice.h>
 
 #if 0
@@ -581,7 +582,7 @@
 			device->interface = le16_to_cpu (request->wIndex);
 			device->alternate = le16_to_cpu (request->wValue);
 			/*dbg_ep0(2, "set interface: %d alternate: %d", device->interface, device->alternate); */
-			serial_printf ("DEVICE_SET_INTERFACE.. event?\n");
+			serial_printf("DEVICE_SET_INTERFACE.. event?\n");
 			return 0;
 
 		case USB_REQ_GET_STATUS: