omap3_beagle: Flush UART3 xmit on enable if TEMT is broken

Flush UART3 xmit on enable if TEMT is broken

On some OMAP3 devices when UART3 is configured for boot mode before SPL starts
only THRE bit is set. We have to empty the transmitter before initialization
starts. This patch avoids the use of CONFIG_SYS_NS16550_BROKEN_TEMT.

Signed-off-by: Manfred Huber <man.huber@arcor.de>
Tested-by: Javier Martinez Canillas <javier@dowhile0.org>
Tested-by: Andreas Bießmann <andreas.devel@googlemail.com>