forcedeth: replace pci_unmap_page with dma_unmap_page

The function pci_unmap_page is obsolete. So it is replaced with
the function dma_unmap_page.

CC: Srinivas Eeda <srinivas.eeda@oracle.com>
CC: Joe Jin <joe.jin@oracle.com>
CC: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>