net/dsa/dsa.c: remove unnecessary null test before kfree

Fix checkpatch warning:
WARNING: kfree(NULL) is safe this check is probably not required

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: netdev@vger.kernel.org
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>