Bug fix for asuswrt device_tracker. Issue #3015

[Danielhiversen]

Description:
Should fix issue #3015. Confirmed to solve the issue by gitter user; https://gitter.im/home-assistant/home-assistant?at=57c2feab5b11de77215af391

Related issue (if applicable): fixes #3015

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#

Example entry for configuration.yaml (if applicable):

Checklist:

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If code communicates with devices, web services, or a:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.