i40e: fix panic due to too-early Tx queue enable

This fixes the panic under traffic load when resetting.  This issue
could also show up if/whenever there is a Tx-timeout.

Change-ID: Ie393a1f17fd5d962e56fc3bfe784899ef25402f5
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>