hw/char/stm32f2xx_usart: fix TXE/TC bit handling

hw/char/stm32f2xx_usart: fix TXE/TC bit handling

I/O currently being synchronous, there is no reason to ever clear the
SR_TXE bit. However the SR_TC bit may be cleared by software writing
to the SR register, so set it on each write.

In addition, fix the reset value of the USART status register.

Signed-off-by: Richard Braun <rbraun@sceen.net>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
[PMM: removed XXX tag from comment, since it isn't something
 we need to come back and fix in QEMU]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>