switchdev: mrp: Remove the variable mrp_ring_state

Remove the variable mrp_ring_state from switchdev_attr because is not
used anywhere.
The ring state is set using SWITCHDEV_OBJ_ID_RING_STATE_MRP.

Fixes: c284b545 ("switchdev: mrp: Extend switchdev API to offload MRP")
Acked-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>