dmm/bm86x: check more bytes in DMM packet (undocumented by vendor)

dmm/bm86x: check more bytes in DMM packet (undocumented by vendor)

User johnch reported via IRC that bm86x meters can lose sync to the
packet stream which goes unnoticed and tries to process invalid data.

Tighten the packet validity check, compare four bytes of the third chunk
in the DMM packet. This improves reliability, but isn't backed by vendor
documentation. It's yet to get determined whether other existing devices
don't provide the four-byte magic pattern.