ixgb: remove lltx support and update tx routine

a) kernel developers suggest LLTX is broken and unsafe to use, remove it.
b) remember to pre-stop the queue if we won't have room
c) removing lltx means we can remove our tx lock

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>