sfc: Don't use efx_filter_{build,hash,increment}() for default MAC filters

These functions happen to work for default MAC filters: they generate
an initial index of 1/0 for unicast/multicast respectively and an
increment of 1 for either, so a search succeeds at depth 2.  But this
is a matter of luck rather than design, and it really won't work well
with the bug fix we're about to do.

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