staging:iio:buffer - remove unused event code for buffer events.

These events are no longer passed up to userspace.

ad7745 - was using these events to indicate directly dataready
events.  I'm not sure when it ever makes sense to push these
to userspace so for now I've taken them to event codes 0 and 1
until someone has time to make this driver do something more
standard. (they were arbitary before, they still are, be it
in a different way!)

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