serial: qcom-geni: fix DMA mapping leak on shutdown

Fix what appears to be a copy-paste error that can lead to a leaked DMA
mapping on close() and failure to restart TX after the port is reopened.

Note that rx_dma_addr is generally NULL when
qcom_geni_serial_stop_tx_dma() is called as part of shutdown() (but
tx_dma_addr need not be).

Fixes: 2aaa43c7

 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Cc: stable <stable@kernel.org>
Cc: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Tested-by: Andrew Halaney <ahalaney@redhat.com> # sa8540p-ride
Link: https://lore.kernel.org/r/20230307164405.14218-3-johan+linaro@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>