sfc: Fix the reported priorities of different filter types

Each RX filter table contains filters with two different levels of
specificity: TCP/IPv4 and UDP/IPv4 filters match the local address and
port and optionally the remote address and port; Ethernet filters
match the local address and optionally the VID.  The more specific
filters always override less specific filters within the same table,
and should be numbered accordingly.

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