sfc: Fix lookup of default RX MAC filters when steered using ethtool

commit 385904f819e3 ('sfc: Don't use
efx_filter_{build,hash,increment}() for default MAC filters') used the
wrong name to find the index of default RX MAC filters at insertion/
update time.  This could result in memory corruption and would in any
case silently fail to update the filter.

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