staging:iio:ad7606: Let the common probe function return int

The common probe function for the ad7606 currently returns a struct iio_dev
pointer. The returned value is not used by the individual driver probe
functions other than for error checking.

Let the common probe function return a int instead to report the error
value directly (or 0 on success). This allows to simplify the code.

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