bnx2: remove skb_dma_map/unmap calls from driver

Due to the fact that skb_dma_map/unmap do not work correctly when a HW
IOMMU is enabled it has been recommended to go about removing the calls
from the network device drivers.

[ Fix bnx2_free_tx_skbs() ring indexing and use NETDEV_TX_OK return
  code in bnx2_start_xmit() after cleaning up DMA mapping errors. -Mchan ]

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>