staging:iio:ad7793: Implement stricter id checking

Instead of checking whether the id of the current device matches the id of any
device supported by the driver, check whether it matches the id of the device
which the driver was instantiated for. This makes sure that the driver is not
accidentally instantiated for the wrong device.

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