net/enic: use 64B completion queue entries if available

net/enic: use 64B completion queue entries if available

Latest VIC adapters support 64B CQ (completion queue) entries as well
as 16B entries available on all VIC models. 64B entries can greatly
reduce cache contention (CPU stall cycles) between DMA writes (Rx
packet descriptors) and polling CPU. The effect is very noticeable on
Intel platforms with DDIO. As most UCS servers are based on Intel
platforms, enable and use 64B CQ entries by default, if
available. Also, add devarg 'cq64' so the user can explicitly disable
64B CQ.

Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>