netxen: fix tx ring accounting

This forces every update of tx ring producer to check for
availability of space for next full TSO command. Earlier
firmware control commands didn't care to pause tx queue.

Stop the tx queue if there's not enough space to transmit one full
LSO command left on the tx ring after current transmit. This avoids
returning NETDEV_TX_BUSY after checking distance between producer
and consumer on every cpu.

Restart the tx queue only if we have cleaned up enough tx
descriptors.

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>