staging: iio: dac: more consistent DAC sysfs attributes naming

Align DAC sysfs attributes naming with the convention used by ADC

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