sfc: More sensible semantics for efx_filter_insert_filter() replace flag

The 'replace' flag to efx_filter_insert_filter() controls whether the
new filter may replace *any* filter, and is checked even before
priority comparison.  But lower-priority filters should never
block insertion of higher-priority filters.

Change the priority checking so that lower-priority filters are
replaced regardless of the value of the flag, and rename the
flag to 'replace_equal'.

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