serial: qcom-geni: fix shutdown race

A commit adding back the stopping of tx on port shutdown failed to add
back the locking which had also been removed by commit e8376633
("tty: serial: qcom_geni_serial: No need to stop tx/rx on UART
shutdown").

Holding the port lock is needed to serialise against the console code,
which may update the interrupt enable register and access the port
state.

Fixes: d8aca2f9 ("tty: serial: qcom-geni-serial: stop operations in progress at shutdown")
Fixes: 947cc4ec ("serial: qcom-geni: fix soft lockup on sw flow control and suspend")
Cc: stable@vger.kernel.org	# 6.3
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20241009145110.16847-4-johan+linaro@kernel.org


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