ethtool: make ethtool_ops::cap_rss_ctx_supported optional

cap_rss_ctx_supported was created because the API for creating
and configuring additional contexts is mux'ed with the normal
RSS API. Presence of ops does not imply driver can actually
support rss_context != 0 (in fact drivers mostly ignore that
field). cap_rss_ctx_supported lets core check that the driver
is context-aware before calling it.

Now that we have .create_rxfh_context, there is no such
ambiguity. We can depend on presence of the op.
Make setting the bit optional.

Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>