ixgb: move time stamp set before setting dma pointer

a user pointed out that setting variables out of order with respect
to the checks we make for tx timeout handling could result in a race
where ->dma was set but ->time_stamp was set to the old value.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>