netmap: bridge: fix transmission in busy-wait mode

In busy-wait mode (BUSYWAIT defined), NIOCTXSYNC should be
performed after packets have been moved to the TX ring
(rather than before).
Before the change, moved packets may stall for an indefinite
time in the TX ring.

MFC after: 1 week

(cherry picked from commit 51cc31088bf4d23a6ad0bfe8851adaa049d750fc)