iio: adc: exynos_adc: fix OF populate on driver rebind

iio: adc: exynos_adc: fix OF populate on driver rebind

Since commit c6e126de43e7 ("of: Keep track of populated platform
devices") child devices will not be created by of_platform_populate()
if the devices had previously been deregistered individually so that the
OF_POPULATED flag is still set in the corresponding OF nodes.

Switch to using of_platform_depopulate() instead of open coding so that
the child devices are created if the driver is rebound.

Fixes: c6e126de43e7 ("of: Keep track of populated platform devices")
Cc: stable@vger.kernel.org	# 3.16
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>