i40e: fix compile warning on checksum_local

Compile testing with higher warning levels found this complaint:
i40e_nvm.c: warning: 'checksum_local' may be used uninitialized in
this function

Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>