iio: iio_buffer_register: Use correct channel when calculating masklength

The channel set assigned to the iio device is not necessarily the same has the
channel set passed to iio_buffer_register. So to avoid possible complications
always work with the channel set pass to iio_buffer_register and ignore the
channel set assigned to the iio device.

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