Avoid Sonos error when joining with self

[amelchio]

Description:

This fixes an error that could happen if we attempted to add an entity as its own slave.

2018-03-13 23:50:22 ERROR (SyncWorker_19) [homeassistant.components.media_player.sonos] Error on join with UPnP Error 402 received: Invalid Args from 10.0.2.14

Related issue (if applicable): reported in forum

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox.

If the code communicates with devices, web services, or third-party tools:

• 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.