BCM7278 has only 128 entries while BCM7445 has the full 256 entries set,
fix that.

Fixes: 7318166c ("net: dsa: bcm_sf2: Add support for ethtool::rxnfc")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>