i40e: verify string count matches even on early return

Similar to i40e_get_ethtool_stats, add a goto to verify that the data
pointer for the strings lines up with the expected stats count. This
helps ensure that bugs are not introduced when adding stats.

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>