AT91: Small fix on AT91 USART initialization code

Before reset dbgu transmitter, we just wait TXEMPTY to drain the
transmitter register(Just in case). If not doing this, we may sometimes
see several weird characters from DBGU.

A short delay is also added to make sure the new serial settings are
settled.

Signed-off-by: Hong Xu <hong.xu@atmel.com>
[cherry-picked from u-boot-atmel/old-next]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>