serial: stm32: Give H/W a chance to set the TXE bit on transfer

It is possible to evaluate the TXE bit *before* H/W has had the
opportunity to detect that data is being processed.  Therefore
we should hold off on any evaluation until TXE has initially been
set.

Change-Id: Iff26bfbe3ab419734003bf81a4cb357de83908e7
Signed-off-by: Lee Jones <lee.jones@linaro.org>