net/mlx5e: Enable stateless offloads for VF representor netdevs

Enabled checksum and TSO offloads for the representors, in
order to increase their performance, which is required to
increase the performance of flows that cannot be offloaded.

Checksum offloads contribute to a general acceleration of all
traffic (to around 150%), whereas the TSO offload contributes
to a prominent acceleration of the representor's TX for traffic
flows with larger than MTU sized packets (to around 200%). This
is the usual case for TCP streams, as the PF, which serves as
the uplink representor, and the VF representors employ GRO before
forwarding the packets to the representor.

GRO was enabled implicitly for the representors beforehand, and
is explicitly enabled here to ensure that the representors preserve
the performance boost it provides (of around 200%) when working in
tandem with the TSO offload by the forwardee, which is the standard
case as both the PF and the VF representors employ HW TSO.

The impact of these changes can be seen in the following
measurements taken on a setup of a VM over a VF, connected
to OVS via the VF representor, to an external host:

Before current changes:
                     TCP Throughput [Gb/s]
External host to VM         ~ 10.5
VM to external host         ~ 23.5

With just checksum offloads enabled:
                     TCP Throughput [Gb/s]
External host to VM         ~ 14.9
VM to external host         ~ 28.5

With the TSO offload also enabled:
                     TCP Throughput [Gb/s]
External host to VM         ~ 30.5

Signed-off-by: Gavi Teitz <gavi@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>