i40e: calculate ethtool stats size in a separate function

Use a separate function to calculate the number of stats for
a particular device. This helps reduce the clutter in
i40e_get_sset_count().

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>