hw/char: cadence_uart: Ignore access when unclocked or in reset for uart_{read, write}()

hw/char: cadence_uart: Ignore access when unclocked or in reset for uart_{read, write}()

Read or write to uart registers when unclocked or in reset should be
ignored. Add the check there, and as a result of this, the check in
uart_write_tx_fifo() is now unnecessary.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210901124521.30599-6-bmeng.cn@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>