genericmiot: handle write-only settings

[rytilahti]

Write-only settings generate also setting descriptors with property names.
This is confusing as the property is not really available in the status container, causing KeyError on access.

Potential solutions:

• Add access information to the settings descriptor
• Set property to None and leave it for downstreams to check for that

[rytilahti]

Already done in #1662.