net/mlx5e: Add IPv6 traffic class (DSCP) header rewrite support

Add support for rewriting of IPV6 DSCP part of traffic class field.
Next commands, for example, can be used to offload rewrite action:

OVS:
 $ ovs-ofctl add-flow ovs-sriov "tcpv6, in_port=REP, \
       actions=mod_nw_tos:68, output:NIC"

iproute2:
 $ tc filter add dev REP ingress protocol ipv6 prio 1 flower skip_sw \
       ip_proto tcp \
       action pedit ex munge ip6 traffic_class set 68 retain 0xfc pipe \
       action mirred egress redirect dev NIC

Signed-off-by: Maor Dickman <maord@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>