acenic: fix the misusage of zero dma address

acenic 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>
Signed-off-by: David S. Miller <davem@davemloft.net>