Insteon: Fix Error in PLM Message Integrity Check Logic

Fix bugs in which PLM commands with a letter were rejected as being too long. Notably this prevented the scanning of the PLM link table.

Also fix bug which prevented MH from sending extended length messages to the PLM.

Fixes Issues
hollie/misterhouse#170
mstovenour/misterhouse#14
Merge pull request #171 from krkeegan/fix_issue_83

Insteon: Fix Error in PLM Message Integrity Check Logic