i40e: clean up local variable initialization

In both i40e_calc_nvm_checksum() and i40e_update_nvm_checksum(), the
local variables designated by 'ret_code' are overwritten immediately. As
such, they should merely be declared.

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>