sfc: Merge efx_filter_search() into efx_filter_insert()

efx_filter_search() is only called from efx_filter_insert(), and
neither function is very long.  The following bug fix requires a more
sophisticated search with a third result, which is going to be easier
to implement as part of the same function.

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