ixgb: maybe stop tx port missed a piece

back when maybe stop tx was added to the ixgb driver some mistakes
were made and the driver
a) didn't remove the tx lock, which is now un-necessary
b) didn't change the restart code to be compliant with maybe_stop

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>