Fixed missing Interface attribute required for BACnet COV.

[davidraker]

Description

The device_path is passed from the DriverAgent to the Inteface on initialization, but was not being saved. It is a required argument to BACnetProxy RPC "create_cov_subscription." Added self.device_path variable in BaseInterface.

Fixes #2484

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

This has been tested on real hardware.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules