net/ice: update writeback policy to reduce latency

net/ice: update writeback policy to reduce latency

Just like iavf, setting the value to 2us allows for generally good
streaming packet performance while keeping latency down, and
generally keeps the performance of the PF and VF interfaces similar.

The previous value of 0x10 was making latency on a single packet
receive be as much as 16us.

Fixes: 65dfc889d86b ("net/ice: support Rx queue interruption")
Cc: stable@dpdk.org
Reported-by: Brian Johnson <brian.johnson@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>