i40e: split i40e_get_strings() into smaller functions

Split the statistic strings and private flags strings into their own
separate functions to aid code readability.

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>