spi: spi-cadence: enable SPI_CONTROLLER_MUST_TX

spi: spi-cadence: enable SPI_CONTROLLER_MUST_TX

During an SPI read operation, even if the xspi->txbuf passed to the
cdns_spi_writerinterface is empty, it is still necessary to call
cdns_spi_write(xspi, CDNS_SPI_TXD, txw); otherwise, the read operation
will fail to obtain data correctly due to a lack of clocks.

Fixes: 4e00135b2dd1 ("spi: spi-cadence: supports transmission with bits_per_word of 16 and 32")
Reported-by: Rodrigo Alencar <455.rodrigo.alencar@gmail.com>
Closes: https://lore.kernel.org/all/lbijvnnwsnddonmm5pveqzap6iibxhl4maneq43x4j6w64dev6@u75qhm5cwiob/
Signed-off-by: Jun Guo <jun.guo@cixtech.com>
Link: https://patch.msgid.link/20260115091924.844179-1-jun.guo@cixtech.com
Signed-off-by: Mark Brown <broonie@kernel.org>