staging:iio:impedance-analyser make use of iio_sw_buffer_preenable

This avoids some code duplication by using the generic form
in a non performance critical place.

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>