sfc: Generalise filter spec initialisation

Move search_depth arrays into per-table state.

Define initialisation function efx_filter_init_rx() which sets
everything apart from the match fields.

Define efx_filter_set_{ipv4_local,ipv4_full,eth_local}() to set the
match fields.  This allows some simplification of callers and later
support for additional protocols and more flexible matching using
multiple calls to these functions.

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