i40e: check for netdev before debugfs use

Make sure the VSI has a netdev before trying to use it
in the debugfs netdev_ops commands.

Change-ID: I2d744fc0c32b3226534ce2cde171d9675c5440a6
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>