ixgbe: inline the ixgbe_maybe_stop_tx function

ixgbe: inline the ixgbe_maybe_stop_tx function

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>
(cherry picked from commit 82d4e46e2a398154273044dd9813206f0d85bc09)

Signed-off-by: Joe Jin <joe.jin@oracle.com>