ixgbe: Reformat output of ixgbe_dump

Reformats the output of the Tx/Rx descriptor dumps to more
appropriately align the output of the ixgbe_dump and improve readability.
Prevents empty Tx descriptors from being displayed to decrease the size
of the dump and make it more manageable.

Signed-off-by: Josh Hay <joshua.a.hay@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>