staging;iio:adc:ad7606 use iio_sw_buffer_preenable rather than local equiv

Other than a few slight refactorings the local version was pretty standard
so replace it and rework to get rid of st->d_size which it setup.

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