hw/char: cadence_uart: Move clock/reset check to uart_can_receive()

hw/char: cadence_uart: Move clock/reset check to uart_can_receive()

Currently the clock/reset check is done in uart_receive(), but we
can move the check to uart_can_receive() which is earlier.

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: 20210901124521.30599-4-bmeng.cn@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>