staging: fwserial: fix checkpatch permission warnings

Fix the following warnings:
Symbolic permissions are not preferred. Consider using octal permissions.
Signed-off-by: Simon Chopin <chopin.simon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fwserial/fwserial.c

@@ -39,9 +39,9 @@ static int num_ttys = 4;	    /* # of std ttys to create per fw_card    */
 static bool auto_connect = true;    /* try to VIRT_CABLE to every peer        */
 static bool create_loop_dev = true; /* create a loopback device for each card */
 
-module_param_named(ttys, num_ttys, int, S_IRUGO | S_IWUSR);
-module_param_named(auto, auto_connect, bool, S_IRUGO | S_IWUSR);
-module_param_named(loop, create_loop_dev, bool, S_IRUGO | S_IWUSR);
+module_param_named(ttys, num_ttys, int, 0644);
+module_param_named(auto, auto_connect, bool, 0644);
+module_param_named(loop, create_loop_dev, bool, 0644);
 
 /*
  * Threshold below which the tty is woken for writing