spi_mcux_dspi: Hotfix for failing null rx test

This is not really high priority to support and is disrupting test
reporting, for now just hotfix the test by returning that this is not
supported, it's not really necessary anyways, there's multiple ways to
call the API to achieve the same effect.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>

Author: decsny

drivers/spi/spi_mcux_dspi.c

@@ -687,13 +687,21 @@ static int transceive(const struct device *dev,
 	SPI_Type *base = config->base;
 #endif
 
+	if (rx_bufs == NULL) {
+		/* FIXME: for some reason this messes up the DMA configuration
+		 * probably because CITER is 0 and transfer is starting for some reason
+		 */
+		return -ENOTSUP;
+	}
+
 	spi_context_lock(&data->ctx, asynchronous, cb, userdata, spi_cfg);
 
 	ret = spi_mcux_configure(dev, spi_cfg);
 	if (ret) {
 		goto out;
 	}
 
+
 	spi_context_buffers_setup(&data->ctx, tx_bufs, rx_bufs, 1);
 
 	spi_context_cs_control(&data->ctx, true);
@@ -726,6 +734,7 @@ static int transceive(const struct device *dev,
 	}
 
 	ret = spi_context_wait_for_completion(&data->ctx);
+
 out:
 	spi_context_release(&data->ctx, ret);