staging:iio:adc:ad7291 remove abuse of buffer events and replace with something almost sane

This device has separate events for a sort of decaying average and for
the raw value.  We don't have a way of specifying this as yet.
For now I have both resulting in the same event code.

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