i40e: restrict diag test messages

Use the netif_info() macro to restrict messaging to when the HW
bit is enabled in the msglvl netdev message mask.

Change-Id: I83030d4402991cfb7da100da00f05ce502ada4ae
Signed-off-by: Shannon Nelson <shannon.nelson@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>