octeon_ep: get max rx packet length from firmware

Max receive packet length can vary across SoCs, so
this needs to be queried from respective firmware and
filled by driver. A control net get mtu api should be
implemented to do the same.

Signed-off-by: Shinas Rasheed <srasheed@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>