iio: exynos_adc: do a reinit_completion before the conversion

Add reinit_completion() before the wait_for_completion_timeout in
raw_read() call.

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>