dcb: fix return type on dcb_setapp()

Incorrect return type on dcb_setapp() this routine
returns negative error codes. All call sites of
dcb_setapp() assign the return value to an int already
so no need to update drivers.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>