tty: xilinx_uartps: Prevent writes when the controller is disabled

Prevent writing to the fifo if the controller is disabled.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Link: https://lore.kernel.org/r/20220729114748.18332-6-shubhrajyoti.datta@xilinx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>