ixgbe: added netdev_ops file to debugfs

Added the netdev_ops file to debugfs with a command to call the
ndo_tx_timeout function to give users the ability to simulate a
tx_timeout call made by the kernel.

Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>