staging:iio:events: use IIO_<TYPE> and IIO_MOD_<> instead of IIO_EV_CLASS_<TYPE> etc

The original definitions were duplicated to reduce tree churn during introduction of chan_spec
registration.  Now there is no point in maintaining the two sets of definitions.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>