iio: Update iio_channel_get_all and iio_channel_get_all_cb API

Pass device pointer instead of device name as parameter to iio_channel_get_all
and iio_channel_get_all_cb. This will enable us to use OF information to
retrieve consumer channel information.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>