iio: ssp: use PLATFORM_DEVID_NONE

Use PLATFORM_DEVID_NONE define instead of "-1" value because:
 - it brings some meaning,
 - it might point attention why auto device ID was not used.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200921204939.20341-1-krzk@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>