ixgbe: fix ethtool loopback diagnostic with DCB enabled

This patch disables DCB prior to running the loopback test.
When DCB is enabled the frames may be modified on Tx (by adding vlan tag)
which will fail the check on Rx.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Jack Morgan <jack.morgan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>