iio: adis16400: Compute the scan mask from channel indices

We unfortunately can't use ~0UL for the scan mask to indicate that the
only valid scan mask is all channels selected. The IIO core needs the exact
mask to work correctly and not a super-set of it. So calculate the masked
based on the channels that are available for a particular device.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Fixes: 5eda3550 ("staging:iio:adis16400: Preallocate transfer message")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>