hw/char/cmsdk-apb-uart.c: Correctly clear INTSTATUS bits on writes

hw/char/cmsdk-apb-uart.c: Correctly clear INTSTATUS bits on writes

The CMSDK APB UART INTSTATUS register bits are all write-one-to-clear.
We were getting this correct for the TXO and RXO bits (which need
special casing because their state lives in the STATE register),
but had forgotten to handle the normal bits for RX and TX which
we do store in our s->intstatus field.

Perform the W1C operation on the bits in s->intstatus too.

Fixes: https://bugs.launchpad.net/qemu/+bug/1760262
Cc: qemu-stable@nongnu.org
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180410134203.17552-1-peter.maydell@linaro.org