IIO: Ingenic JZ47xx: Error check clk_enable calls.

Introduce error checks for the clk_enable calls used in this driver.
As part of the changes, move clk_enable/clk_disable calls out of
ingenic_adc_set_config and into respective logic of its callers.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Tested-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>