net: lan966x: refactor buffer reload function

Now that we store everything in the fdma structs, refactor
lan966x_fdma_reload() to store and restore the entire struct.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>