igb: fix i350 VF Rx

igb: fix i350 VF Rx

A problem is found on i350 VF. We found TX will happen once
per 4 packets. If only 1~3 packets are received, they will
not be forwarded. But the real problem is on RX side. The
reason is the default RX write-back threshold is changed to
4, so every first 3 packets may be hung there.

This patch checks the RX wthresh when setting up the RX
queue, and forces it to be 1, so every packet can be handled
immediately.

Fixes: 4a41c17dba18 ("igb: set default thresholds based on MAC type")

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>