iio: cm32181: Fix read integration time function

In read integration time function, assign 0 to val.
Because, prevent return inaccurate value when call read integration time.
Cc: Stable@vger.kernel.org
Cc: Kevin Tsai <ktsai@capellamicro.com>
Signed-off-by: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>