iio: adc: ad4130: only set GPIO_CTRL if pin is unused

Currently, GPIO_CTRL bits are set even if the pins are used for
measurements.

GPIO_CTRL bits should only be set if the pin is not used for
other functionality.

Fix this by only setting the GPIO_CTRL bits if the pin has no
other function.

Fixes: 62094060

 ("iio: adc: ad4130: add AD4130 driver")
Signed-off-by: Cosmin Tanislav <demonsingur@gmail.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240207132007.253768-2-demonsingur@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>