Make parameter debug writable

Overseen in the last patch series.
Signed-off-by: Karsten Keil <kkeil@suse.de>

drivers/isdn/hardware/mISDN/hfcpci.c

@@ -61,7 +61,7 @@ u32	hfc_jiffies;
 
 MODULE_AUTHOR("Karsten Keil");
 MODULE_LICENSE("GPL");
-module_param(debug, uint, 0);
+module_param(debug, uint, S_IRUGO | S_IWUSR);
 module_param(poll, uint, S_IRUGO | S_IWUSR);
 
 enum {