term/serial: Improve detection of duplicate serial ports

We currently rely on some pretty fragile comparison by name to
identify whether a serial port being configured is identical

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/ieee1275/console.h

@@ -28,7 +28,7 @@ void grub_console_init_lately (void);
 /* Finish the console system.  */
 void grub_console_fini (void);
 
-const char *
+struct grub_serial_port *
 grub_ofserial_add_port (const char *name);
 
 #endif /* ! GRUB_CONSOLE_MACHINE_HEADER */