i40e: Revert ShadowRAM checksum calculation change

The reason of this revert is unexpected issue found in NVM Update tool
during NVM image downgrade. The implementation is no longer needed
since the QV tools are already aware of new FW double ShadowRAM dump
mechanism.

This patch reverts ShadowRAM checksum calculation change introduced in
commit 9d12f0c4e436 ("i40e: Revert ShadowRAM checksum calculation change")

Signed-off-by: Maciej Paczkowski <maciej.paczkowski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>