e1000: remove the check: skb->len<=0

There is no case skb->len would be 0 or 'negative'.
Remove the check.

Signed-off-by: Yongjian Xu <xuyongjiande@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>