sh_eth: remove checks around dev_kfree_skb() calls

Since consume_skb() (and hence dev_kfree_skb() macro) checks the passed pointer
for NULL, there's no need to check for NULL before invoking dev_kfree_skb().

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>