net/mlx5e: Claim TC hw offloads support only under a proper build config

Currently, we are only supporting tc hw offloads when the eswitch
support is compiled in, but we are not gating the adevertizment
of the NETIF_F_HW_TC feature on this config being set.

Fix it, and while doing that, also avoid dealing with the feature
on ethtool when the config is not set.

Fixes: e8f887ac ('net/mlx5e: Introduce tc offload support')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>