staging:iio:core simplify and fix a bug in iio_device_(un)register_event_set.

The unwinding index was j, not i and it is much simpler to do these as a single
loop unwinding those elements necessary in before jumping to the error handler.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>