iio: adc: ad7768-1: remove tx_buf from IIO buffer read transfer

The address byte is not needed in the SPI transfer if continuous
read mode is enabled. In this case, data readback occurs on the
application of SCLK.

Remove tx_buf from IIO buffer read transfer to reduce the duration of
each transfer and enable the sample rate of 1.024 MHz at 16-bit
resolution, given the maximum supported SCLK rate of 20 MHz:

- With address byte: (16-bit data + 8-bit address) * 1.024 MHz > 20 MHZ.
  Insufficient SCLK cycles to drive out the output.

- Without address byte: (16-bit data) * 1.024 MHz < 20 MHz.

Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>

Author: jonathanns

drivers/iio/adc/ad7768-1.c

@@ -1141,7 +1141,6 @@ static int ad7768_buffer_postenable(struct iio_dev *indio_dev)
 		.len = 1,
 	};
 	unsigned int rx_data[2];
-	unsigned int tx_data[2];
 	struct spi_message msg;
 	int ret;
 
@@ -1163,8 +1162,6 @@ static int ad7768_buffer_postenable(struct iio_dev *indio_dev)
 	if (st->spi_is_dma_mapped) {
 		spi_bus_lock(st->spi->master);
 
-		tx_data[0] = AD7768_RD_FLAG_MSK(AD7768_REG_ADC_DATA) << 24;
-		xfer.tx_buf = tx_data;
 		xfer.rx_buf = rx_data;
 		spi_message_init_with_transfers(&msg, &xfer, 1);
 		ret = spi_engine_offload_load_msg(st->spi, &msg);