i40e: refactor stats collection

Pull the PF stat collection out of the VSI collection routine, and
add a unifying stats update routine to call the various stat collection
routines.

Change-ID: I224192455bb3a6e5dc0a426935e67dffc123e306
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>