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