i40e: restrict diag test length

The diagnostics test needs some slightly different limits
in order to succeed.

Change-Id: Ia1c49148af92fa4be20778a819f69350381bf865
Signed-off-by: Kamil Krawczyk <kamil.krawczyk@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>