iio: adc: max9611: Fix misuse of GENMASK macro

Arguments are supposed to be ordered high then low.

Signed-off-by: Joe Perches <joe@perches.com>
Fixes: 69780a3b ("iio: adc: Add Maxim max9611 ADC driver")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>