sfc: Extend and abstract efx_filter_spec to cover Huntington/EF10

Replace type field with match_flags.  Add rss_context and match values
covering of most of what is now in the MCDI protocol.

Change some fields into bitfields so that the structure size doesn't grow
beyond 64 bytes.

Ditch the filter decoding functions as it is now easier to pick apart
the abstract structure.

Rewrite ethtool NFC rule functions to set/get filter match flags and
values directly.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>