TTY: amiserial, add missing platform check

When booting a multi-platform m68k kernel on a non-Amiga with
"console=ttyS0" on the kernel command line, it crashes with:

Unable to handle kernel access at virtual address 81dff01c
Oops: 00000000
PC: [<001e09a8>] serial_console_write+0xc/0x70

Add the missing platform check to amiserial_console_init() to fix this.
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/amiserial.c

@@ -1855,6 +1855,9 @@ static struct console sercons = {
  */
 static int __init amiserial_console_init(void)
 {
+	if (!MACH_IS_AMIGA)
+		return -ENODEV;
+
 	register_console(&sercons);
 	return 0;
 }