cadence_uart: Flush queued characters on reset

cadence_uart: Flush queued characters on reset

Reset can be used to empty the rx-fifo. As the fifo full condition is
used to return false from can_receive, queued rx data should be flushed
on reset accordingly.

Cc: Wendy Liang <jliang@xilinx.com>
Cc: Jason Wu <huanyu@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reported-by: Jason Wu <huanyu@xilinx.com>
Message-id: 494c1e005e225c915d295ddfd75d992ad2dabc3c.1364964526.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>