staging:iio:adc:ad7745 move from old to current event handling.

Nice simple one.  Not sure we actually want devices outputting
datardy signals like that, but I'll leave it for now.
Cleaned up some unneeded functions whilst here.

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