TEMT is set when the transmitter is totally empty and all output has
finished.

This prevents output problems (including a loss of synchronization
observed on p2020 that persisted for quite a while) if SPL has output
still on its way out.

Signed-off-by: Scott Wood <scottwood@freescale.com>
--
v2: fixed typo in subject, and explained what the bit does in the changelog