serial: st-asc: Don't call BUG in asc_console_setup()

In order to prevent an asc instance to be used as early console, BUG_ON is
used on either mapbase or membase being NULL.

Problem is that this condition is also true when we set console to be a ttyASx
different to the first asc instance being probed.

Instead of calling BUG_ON, it now returns -ENXIO when either mapbase or
membase is NULL.
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/st-asc.c

@@ -842,7 +842,8 @@ static int asc_console_setup(struct console *co, char *options)
 	 * this to be called during the uart port registration when the
 	 * driver gets probed and the port should be mapped at that point.
 	 */
-	BUG_ON(ascport->port.mapbase == 0 || ascport->port.membase == NULL);
+	if (ascport->port.mapbase == 0 || ascport->port.membase == NULL)
+		return -ENXIO;
 
 	if (options)
 		uart_parse_options(options, &baud, &parity, &bits, &flow);