gianfar: correct filer table writing

MAX_FILER_IDX is the last usable index.  Using less-than
will already guarantee that one entry for catch-all rule
will be left, no need to subtract 1 here.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>