iio: exynos_adc: reduce timeout and use wait_for_completion_timeout

ADC module on Exynos5 SoCs runs at 600KSPS. At this conversion rate,
waiting for 1000 msecs is wasteful (incase of h/w failure).

Hence, reduce the time out to 100msecs and use
wait_for_completion_timeout() instead of
wait_for_completion_interruptible_timeout()

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>