ixgbe: no need for esp trailer if GSO

There is no need to calculate the trailer length if we're doing
a GSO/TSO, as there is no trailer added to the packet data.
Also, don't bother clearing the flags field as it was already
cleared earlier.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>