ATcommandControlledDiagInterface::write(): fix buffer size check for SSM2 via ISO-14230

ISO14230-SSM2 messages are at least 6 bytes long (4 header bytes + data + 1 checksum byte)
ATcommandControlledDiagInterface::write(): remove checksum byte from ISO14230-SSM2 messages

ISO14230-SSM2 messages are passed to the interface with header and checksum byte.
But the interface automatically adds both, so the checksum byte also has to be
removed before passing the message to the interface.