openvswitch: Set internal device max mtu to ETH_MAX_MTU.

Commit 91572088 ("net: use core MTU range checking in core net
infra") changed the openvswitch internal device to use the core net
infra for controlling the MTU range, but failed to actually set the
max_mtu as described in the commit message, which now defaults to
ETH_DATA_LEN.

This patch fixes this by setting max_mtu to ETH_MAX_MTU after
ether_setup() call.

Fixes: 91572088 ("net: use core MTU range checking in core net infra")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>