iio:ad7793: Fix ad7785 product ID

While the datasheet for the AD7785 lists 0xXB as the product ID the actual
product ID is 0xX3.

Fix the product ID otherwise the driver will reject the device due to non
matching IDs.

Fixes: e786cc26 ("staging:iio:ad7793: Implement stricter id checking")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>