i40e: fix unconditional execution of cpu_to_le16()

The commit 3092e5e4cc79 ("i40e: add little endian conversion for
checksum") fixed the checksum bug on big-endian architecture.

But we should not execute cpu_to_le16() unconditionally. Thus, put
cpu_to_le16() under certain condition.

Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>
Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>