cxgb4: Add capability to get/set SGE Doorbell Queue Timer Tick

This patch gets/sets SGE Doorbell Queue timer ticks via ethtool

Original work by: Casey Leedom <leedom@chelsio.com>

Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>