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 <[e-mail]>
Acked-by: Karsten Keil <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/isdn/capi/kcapi.c

···
 		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);