hw/char: cadence_uart: Disable transmit when input clock is disabled

hw/char: cadence_uart: Disable transmit when input clock is disabled

At present when input clock is disabled, any character transmitted
to tx fifo can still show on the serial line, which is wrong.

Fixes: b636db306e06 ("hw/char/cadence_uart: add clock support")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20210901124521.30599-3-bmeng.cn@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>