mlx5ib: Fix ethertype to be ETH_P_IPV6

Sponsored by: Juniper Networks, Inc.
MFC after: 7 days
Reviewed by: ae, kib, zlei
Differential Revision: https://reviews.freebsd.org/D42184

(cherry picked from commit 3b173281764ba7c02f9de0a67caa1c4eca604413)