iio: core: Remove IIO_EV_TYPE_INSTANCE

By introducing IIO_EV_TYPE_CHANGE, IIO_EV_TYPE_INSTANCE becomes redundant.
The effect of IIO_EV_TYPE_INSTANCE can be obtained by using IIO_EV_TYPE_CHANGE
with IIO_EV_INFO_VALUE set to 1.

Remove all instances of IIO_EV_TYPE_INSTANCE and replace them with
IIO_EV_TYPE_CHANGE where needed.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>