openvswitch: Fix push/pop ethernet validation

When there are both pop and push ethernet header actions among the
actions to be applied to a packet, an unexpected EINVAL (Invalid
argument) error is obtained. This is due to mac_proto not being reset
correctly when those actions are validated.

Reported-at:
https://mail.openvswitch.org/pipermail/ovs-discuss/2018-October/047554.html
Fixes: 91820da6

 ("openvswitch: add Ethernet push and pop actions")
Signed-off-by: Jaime Caamaño Ruiz <jcaamano@suse.com>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>