net: fman: memac: Use params instead of priv for max_speed

This option is present in params, so use it instead of the fman private
version.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Acked-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>