serial: qcom-geni: fix mapping of empty DMA buffer

Make sure that there is data in the ring buffer before trying to set up
a zero-length DMA transfer.

This specifically fixes the following warning when unmapping the empty
buffer on the sc8280xp-crd:

   WARNING: CPU: 0 PID: 138 at drivers/iommu/dma-iommu.c:1046 iommu_dma_unmap_page+0xbc/0xd8
   ...
   Call trace:
    iommu_dma_unmap_page+0xbc/0xd8
    dma_unmap_page_attrs+0x30/0x1c8
    geni_se_tx_dma_unprep+0x28/0x38
    qcom_geni_serial_isr+0x358/0x75c

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/20230307164405.14218-4-johan+linaro@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>