iio: exynos_adc: rearrange clk and regulator enable/disable calls

This patch maintains the following order in
probe(), remove(), resume() and suspend() calls

regulator enable, clk prepare enable
...
clk disable unprepare, regulator disable

While at it,
1. enable the regulator before the iio_device_register()
2. handle the return values for enable/disable calls

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