net: fman: Change return type of disable to void

When disabling, there is nothing we can do about errors. In fact, the
only error which can occur is misuse of the API. Just warn in the mac
driver instead.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Acked-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>