TTY: mxser, fix invalid module_parm permissions

444 means 0674 and we do not definitely want that. Use S_IRUGO which
is much more safer.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Rusty Russell <rusty@ozlabs.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>