i40e: fix use of untrusted scalar value warning

This is a fix for an issue reported by coverity, reported by
Hannes Frederic Sowa.

I'm unable to test if this patch actually fixes the coverity
reported issue, feedback is welcome.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
CC: Hannes Frederic Sowa <hannes@stressinduktion.org>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>