spi: spi-geni-qcom: enable SPI_CONTROLLER_MUST_TX for GPI DMA mode

spi: spi-geni-qcom: enable SPI_CONTROLLER_MUST_TX for GPI DMA mode

The GPI DMA mode requires for TX DMA to be prepared. Force SPI core to
provide TX buffer even if the caller didn't provide one by setting the
SPI_CONTROLLER_MUST_TX flag.

Fixes: b59c122484ec ("spi: spi-geni-qcom: Add support for GPI dma")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20230629095847.3648597-1-dmitry.baryshkov@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>