serial: qcom-geni: fix false console tx restart

Commit 663abb1a ("tty: serial: qcom_geni_serial: Fix UART hang")
addressed an issue with stalled tx after the console code interrupted
the last bytes of a tx command by reenabling the watermark interrupt if
there is data in write buffer. This can however break software flow
control by re-enabling tx after the user has stopped it.

Address the original issue by not clearing the CMD_DONE flag after
polling for command completion. This allows the interrupt handler to
start another transfer when the CMD_DONE interrupt has not been disabled
due to flow control.

Fixes: c4f52879 ("tty: serial: msm_geni_serial: Add serial driver support for GENI based QUP")
Fixes: 663abb1a ("tty: serial: qcom_geni_serial: Fix UART hang")
Cc: stable@vger.kernel.org	# 4.17
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240906131336.23625-3-johan+linaro@kernel.org


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