ixgbe: xmit frame refactor

ixgbe_xmit_frame can be refactored to use fewer locals and better
utilize common kernel macros.

also fixed minor buglet with internal to driver vlan flag variable being
passed incorrectly.

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>