sfc: Remove filter table IDs from filter functions

The separation between filter tables is largely an internal detail
and it may be removed in future hardware.  To prepare for that:

- Merge table ID with filter index to make an opaque filter ID
- Wrap efx_filter_table_clear() with a function that clears filters
  from both RX tables, which is all that the current caller requires

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