xilinx_uartlite: Accept input after rx FIFO pop

The device return false from the can receive function when the FIFO is
full. This mean the device should check for buffered input whenever a byte is
popped from the FIFO.
Reported-by: NJason Wu <huanyu@xilinx.com>
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>

hw/xilinx_uartlite.c

@@ -97,6 +97,7 @@ uart_read(void *opaque, hwaddr addr, unsigned int size)
                 s->rx_fifo_len--;
             uart_update_status(s);
             uart_update_irq(s);
+            qemu_chr_accept_input(s->chr);
             break;
 
         default: