liquidio: xmit_more support

Defer ringing the Tx doorbell if skb->xmit_more is set unless the Tx queue
is full or stopped.  To keep latency low, use a deferral limit of 8
packets.  We chose 8 because Octeon can fetch at most 8 packets in a single
PCI read, and our tests show that 8 results in low latency.

Signed-off-by: Intiyaz Basha <intiyaz.basha@cavium.com>
Signed-off-by: Satanand Burla <satananda.burla@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>