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 <[e-mail]>
Closes: https://lore.kernel.org/all/lbijvnnwsnddonmm5pveqzap6iibxhl4maneq43x4j6w64dev6@u75qhm5cwiob/
Signed-off-by: Jun Guo <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-cadence.c

···
 	ctlr->unprepare_transfer_hardware = cdns_unprepare_transfer_hardware;
 	ctlr->mode_bits = SPI_CPOL | SPI_CPHA;
 	ctlr->bits_per_word_mask = SPI_BPW_MASK(8);
+	ctlr->flags = SPI_CONTROLLER_MUST_TX;
 
 	if (of_device_is_compatible(pdev->dev.of_node, "cix,sky1-spi-r1p6"))
 		ctlr->bits_per_word_mask |= SPI_BPW_MASK(16) | SPI_BPW_MASK(32);