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

Don't busy-wait in uart_stm32_fifo_fill(), this routine is supposed
to be called on IRQ, and the waiting loop was shown to interfere
with interrupt-driven UART handling.

The original problem mentioned in the patch being reverted ("it is
possible to evaluate the TXE bit *before* H/W has had the
opportunity to detect that data is being processed") may still
exist and may require handling in a different way. The most obvious
way is to replace "while" loop with "if", because STM32 doesn't
have multi-level FIFO anyway. But I was dissuaded to include such a
change as part of this patch, so it's left for future analysis.

This reverts commit 49c2858d.

Change-Id: Ib27b53ba3a29b84c1a2dccff6d33f4118680cc19
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>