char/cadence_uart: Fix reset for unattached instances

commit 1db8b5efe0c2b5000e50691eea61264a615f43de introduced an issue
where QEMU would segfault if you have an unattached Cadence UART.

Fix by guarding the flush-on-reset logic on there being a qemu_chr
attachment.

Reported-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Tested-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Message-id: 9009578ee10a50d994b2e10aa2840d73765f5968.1370577272.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
