Input: ad7879 - fix deficient device disable

Input close or device disable should not interact with the exported
gpiolib functionality. However that's the case. __ad7879_disable()
clears the entire AD7879_REG_CTRL2, while it should just power down
the ADC and its reference.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>