iavf: Fix the math for valid length for ADq enable

There was a calculation error in virtchnl regarding the valid
length which was fixed recently and a corresponding change needs
to go into the code while we enable ADq.

Signed-off-by: Avinash Dayanand <avinash.dayanand@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>