iio: Update iio_channel_get API to use consumer device pointer as argument

For iio_channel_get to work with OF based configurations, it needs the
consumer device pointer instead of the consumer device name as argument.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Anton Vorontsov <anton@enomsg.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>