console: Avoid positive return code from unregister_console()

There are only two callers that use the returned code from
unregister_console():

  - unregister_early_console() in arch/m68k/kernel/early_printk.c
  - kgdb_unregister_nmi_console() in drivers/tty/serial/kgdb_nmi.c

They both expect to get "0" on success and a non-zero value on error.
But the current behavior is confusing and buggy:

  - _braille_unregister_console() returns "1" on success
  - unregister_console() returns "1" on error

Fix and clean up the behavior:

  - Return success when _braille_unregister_console() succeeded
  - Return a meaningful error code when the console was
    not registered before

Link: http://lkml.kernel.org/r/20200203133130.11591-5-andriy.shevchenko@linux.intel.com
To: linux-kernel@vger.kernel.org
To: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/printk.c

@@ -2816,10 +2816,12 @@ int unregister_console(struct console *console)
 		console->name, console->index);
 
 	res = _braille_unregister_console(console);
-	if (res)
+	if (res < 0)
 		return res;
+	if (res > 0)
+		return 0;
 
-	res = 1;
+	res = -ENODEV;
 	console_lock();
 	if (console_drivers == console) {
 		console_drivers=console->next;