iio:imu:adis16400: Sort out missing kernel doc.

I'd like to be enable W=1 for all IIO builds as it catches real issues as well
as more minor documentation issues such as this (also good to fix though!)

drivers/iio/imu/adis16400.c:183: warning: Function parameter or member 'avail_scan_mask' not described in 'adis16400_state'
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200913132115.800131-2-jic23@kernel.org

drivers/iio/imu/adis16400.c

@@ -173,6 +173,8 @@ struct adis16400_chip_info {
  * @variant:	chip variant info
  * @filt_int:	integer part of requested filter frequency
  * @adis:	adis device
+ * @avail_scan_mask:	NULL terminated array of bitmaps of channels
+ *			that must be enabled together
  **/
 struct adis16400_state {
 	struct adis16400_chip_info	*variant;