sfc: Document conditions for multicast replication vs filter replacement

Add the efx_filter_is_mc_recip() function to decide whether a filter
is for a multicast recipient and can coexist with other filters with
the same match values.  Update efx_filter_insert_filter() kernel-doc
to explain the conditions for this.

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