net: dsa: mt7530: Use NULL instead of plain integer

We would be passing 0 instead of NULL as the rsp argument to
mt7530_fdb_cmd(), fix that.

Fixes: b8f126a8 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>