[media] em28xx: prefer_bulk parameter is read-only

As the bulk mode is set at device's probe, it is not possible
to change it later. So, change the parameter to be read only
after modprobing.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/em28xx/em28xx-cards.c

@@ -62,7 +62,7 @@ module_param_array(card,  int, NULL, 0444);
 MODULE_PARM_DESC(card,     "card type");
 
 static unsigned int prefer_bulk;
-module_param(prefer_bulk, int, 0644);
+module_param(prefer_bulk, int, 0444);
 MODULE_PARM_DESC(prefer_bulk, "prefer USB bulk transfers");