isdn/capi: make reset_ctr op truly optional

The CAPI controller operation reset_ctr is marked as optional, and
not all drivers do implement it. Add a check to the kernel CAPI
whether it exists before trying to call it.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Acked-by: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/capi/kcapi.c

@@ -1147,6 +1147,12 @@ load_unlock_out:
 		if (ctr->state == CAPI_CTR_DETECTED)
 			goto reset_unlock_out;
 
+		if (ctr->reset_ctr == NULL) {
+			printk(KERN_DEBUG "kcapi: reset: no reset function\n");
+			retval = -ESRCH;
+			goto reset_unlock_out;
+		}
+
 		ctr->reset_ctr(ctr);
 
 		retval = wait_on_ctr_state(ctr, CAPI_CTR_DETECTED);