staging:iio:adc:ad7887 make use of iio_sw_buffer_preenable.

Using this generic function cuts down on repeated code at the cost
of some computation in a slow path.

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>