staging:iio:adc:ad7476 use iio_sw_buffer_preenable instead of local version.

Now the generic version caches the scan size the version in this driver
is redundant.

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>