i40e: add helper to copy statistic values into ethtool buffer

Similar to the helper function to copy the ethtool stats strings, add
and use a helper function for copying the ethtool stats into the
supplied buffer.

Just like before, we use a macro to avoid having to pass ARRAY_SIZE
manually, so as to reduce chance of bugs.

Some of the stats, especially queue stats, are a bit trickier, and will
be handled in future patches.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>