i40e/virtchnl: refactor code for validate checks

This change updates the arguments passed to the validate function
and fixes the caller, as well as uses the new return values added to
virtchnl.h

One other minor tweak, remove a duplicate set to zero of valid_len.

This is in preparation for moving the function to virtchnl.h.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>