staging:iio:triggers Remove unecessary existence checks and return val

Postenable and predisable are called via buffer->ops so  don't
need to check if buffer exists.

The return value of iio_device_register_trigger_consumer is
always zero and it isn't checked anyway so get rid of it.

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