Input: jz-sadc: Fixed suspend/resume

Thanks to Paul Cercueil for spotting this.
Input: jz-sadc: Cleaned up probe function

Most important change is that input_register_device() is now called at
the very end, when the device is ready to be opened. Thanks to Paul
Cercueil for spotting it was registered too soon.
Input: jz-sadc: Enable interrupt only when cell is enabled

We can't read the touch screen engine if it's disabled. It won't fire
any IRQ then either, but there could be a short window when the engine
is already disabled but the last IRQ not yet handled.
mfd: jz4770-adc: Wait for touch screen engine to shut down

Powering down the SADC while the touch screen engine is not properly
shut down seems to cause problems the next time the engine is enabled.
mfd: jz4770-adc: Split it jz4770_adc_set_enabled()

The amount of shared code between the enable and disable path had
become so small that doing both in a single function was not an
advantage any more.