PLM message parsing wedges if user performs PLM factory reset while connected to MH

[mstovenour]

If the user unplugs the PLM, holds the button, plugs the PLM back in, and continues to hold the button until the PLM quits the long beep while connected to a running MH instance; the messaging parsing logic will get wedged with a PLM message stuck in the receive queue. The PLM send a "user reset detected" message x02x55. Since this is not decoded by the receive loop, the message sticks as "extra" data.