i40e: Don't check for Tx hang when PF down

This patch adds check to bail out if device is already down when checking
for Tx hang subtask.

Change-ID: I3853fb7a6d11cb9a4c349b687cb25c15b19977a0
Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>