ice: safer stats processing

The driver was zeroing live stats that could be fetched by
ndo_get_stats64 at any time. This could result in inconsistent
statistics, and the telltale sign was when reading stats frequently from
/proc/net/dev, the stats would go backwards.

Fix by collecting stats into a local, and delaying when we write to the
structure so it's not incremental.

Fixes: fcea6f3d

 ("ice: Add stats and ethtool support")
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>