hw/char/exynos4210_uart: Fix buffer size reporting with FIFO disabled

hw/char/exynos4210_uart: Fix buffer size reporting with FIFO disabled

Currently the Exynos 4210 UART code always reports available FIFO space
when the backend checks for buffer space. When the FIFO is disabled this
is behavior causes the backend chardev code to replace the data before the
guest can read it.

This patch changes adds the logic to report the capacity properly when the
FIFO is not being used.

Buglink: https://bugs.launchpad.net/qemu/+bug/1913344
Signed-off-by: Iris Johnson <iris@modwiz.com>
Message-id: 20210128033655.1029577-1-iris@modwiz.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>