staging:iio:adc:ad7192 make use of iio_sw_buffer_preenable.

This is not a fast path, so although the original code was more consise
use the generic case to cut down on code repitition.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>