net: fman: Store en/disable in mac_device instead of mac_priv_s

All macs use the same start/stop functions. The actual mac-specific code
lives in enable/disable. Move these functions to an appropriate struct,
and inline the phy enable/disable calls to the caller of start/stop.

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