net: dsa: Remove redundant NULL dst check

tag_lan9303.c does check for a NULL dst but that's already checked by
dsa_switch_rcv() one layer above.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>