iio: adc: ad7606: fix standby gpio state to match the documentation

The binding's documentation specifies that "As the line is active low, it
should be marked GPIO_ACTIVE_LOW". However, in the driver, it was handled
the opposite way. This commit sets the driver's behaviour in sync with the
documentation

Fixes: 722407a4 ("staging:iio:ad7606: Use GPIO descriptor API")
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>