The ixgbe_maybe_stop_tx function is only a few lines long and is called
multiple times through the xmit hotpath.  In order to streamline things it
makes sense to just inline it.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>