be2net: Code changes in Tx path to use skb_dma_map/skb_dma_unmap

Code changes to
 - In the tx completion processing, there were instances of unmapping a
memory as a page which was originally mapped as single. This patch takes care
of this by using skb_dma_map()/skb_dma_unmap() to map/unmap Tx buffers.
 - set gso_max_size to 65535. This was not done till now.

Signed-off-by: Ajit Khaparde <ajitk@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>