staging:iio:ade7758: Use iio_validate_scan_mask_onehot

Only one of the channels of the ade7758 may be sampled at a time. Use the new
validate_scan_mask callback and the iio_validate_scan_mask_onehot function to
implement this restriction. Previously this was implemented using
available_scan_masks, but this requires a individual scan mask for each channel.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>