staging:iio:meter:ad7758 use iio_sw_buffer_preenable to avoid code repition

Using this generic function adds a little overhead to a slow path but
reduces the amount of code repitition in exchange.

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>