kvs40xx: Return NULL from read_data()

Functions called by pthread_create() have the return type (void *).
However, the return statements in read_data() use a value of type
SANE_Status rather than a pointer, which causes a compiler warning.

This return value is never actually used, so return NULL instead.

backend/kvs40xx.c

@@ -542,10 +542,10 @@ static void * read_data (void *arg)
 	sane_get_parameters(s, NULL);
 
 	s->page++;
-	return SANE_STATUS_GOOD;
+	return NULL;
       err:
 	s->scanning = 0;
-	return (void *) st;
+	return NULL;
 }
 
 /* Start scanning */