staging:iio:ad7606: Use oversampling ratio of 1 for no oversampling

Currently the ad7606 driver uses a value of 0 for the oversampling ratio to
express that no oversampling is done. Strictly speaking this means though
that no data capture is done at all. Instead change the driver to use a
value of 1, this is in accordance with what other drivers do and what the
IIO spec suggests.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>