cadance_uart: Accept input after rx FIFO pop

The device returns false from the can receive function when the FIFO is
full. This means the device should check for buffered input whenever a byte is
popped from the FIFO.

Reported-by: Jason Wu <huanyu@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 1360632571-25638-1-git-send-email-peter.crosthwaite@xilinx.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
