benet: fix the misusage of zero dma address

benet driver wrongly assumes that zero is an invalid dma address
(calls dma_unmap_page for only non zero dma addresses). Zero is a
valid dma address on some architectures. The dma length can be used
here.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Sathya Perla <sathyap@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>