be2net: support TX batching using skb->xmit_more flag

This patch uses skb->xmit_more flag to batch TX requests.
TX is flushed either when xmit_more is false or there is
no more space in the TXQ.

Skyhawk-R and BEx chips require an even number of wrbs to be posted.
So, when a batch of TX requests is accumulated, the last header wrb
may need to be fixed with an extra dummy wrb.

This patch refactors be_xmit() routine as a sequence of be_xmit_enqueue()
and be_xmit_flush() calls. The Tx completion code is also
updated to be able to unmap/free a batch of skbs rather than a single
skb.

Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>