STC12A

This is an early STC12 protocol variant; It seems to be used on STC12Cx052 and
possibly other models. It is a mix of STC89 and STC12 protocol versions.

Differences to STC12:

* Uses NONE parity instead of EVEN parity.

* Checksum calculations are different: only a single-byte modular sum is used.

* Baudrate handshake isn't initiated with a type 0x50 packet; this is simply
  skipped.

* After the handshake a special ping-pong sequence of 0x80 type packets needs
  to be sent.
  This doesn't appear to serve any purpose - possibly this is just done to
  verify that the connection works reliably. STC12 doesn't require it anymore
  because parity and the improved checksum ensure correct operation.

* Erase is acknowledged with type 0x80 packet instead of type 0x00

* After flash programming, there is no finish packet, type 0x69, sent; this
  is simply skipped.