iio: adis: add burst_max_speed_hz variable

Typically, in burst mode, the device cannot operate at it's full spi
speed. Hence, the spi transfers for burst mode have to take this into
account. With this change we avoid a potential race with the spi core as
drivers were 'hacking' the device 'max_speed_hz' directly in the
trigger handler.
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210427085454.30616-5-nuno.sa@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/imu/adis_buffer.c

@@ -51,9 +51,13 @@ static int adis_update_scan_mode_burst(struct iio_dev *indio_dev,
 	adis->xfer[0].tx_buf = tx;
 	adis->xfer[0].bits_per_word = 8;
 	adis->xfer[0].len = 2;
+	if (adis->data->burst_max_speed_hz)
+		adis->xfer[0].speed_hz = adis->data->burst_max_speed_hz;
 	adis->xfer[1].rx_buf = adis->buffer;
 	adis->xfer[1].bits_per_word = 8;
 	adis->xfer[1].len = burst_length;
+	if (adis->data->burst_max_speed_hz)
+		adis->xfer[1].speed_hz = adis->data->burst_max_speed_hz;
 
 	spi_message_init(&adis->msg);
 	spi_message_add_tail(&adis->xfer[0], &adis->msg);

include/linux/iio/imu/adis.h

@@ -55,6 +55,7 @@ struct adis_timeout {
  *			this should be the minimum size supported by the device.
  * @burst_max_len:	Holds the maximum burst size when the device supports
  *			more than one burst mode with different sizes
+ * @burst_max_speed_hz:	Maximum spi speed that can be used in burst mode
  */
 struct adis_data {
 	unsigned int read_delay;
@@ -83,6 +84,7 @@ struct adis_data {
 	unsigned int burst_reg_cmd;
 	unsigned int burst_len;
 	unsigned int burst_max_len;
+	unsigned int burst_max_speed_hz;
 };
 
 /**