Change Endian.Big to Endian.BIG as per pymodbus change

[jkjaer]

In https://github.com/pymodbus-dev/pymodbus/pull/1743, pymodbus changed constants.Endian into being an enumeration, resulting in that Endian.Big was changed into Endian.BIG. Unfortunately, this change has broken the EmonModbusTcpInterfacer class which uses Endian.Big for both the byte and word order when calling BinaryPayloadDecoder.fromRegisters (also from pymodbus).

To fix the problem, Endian.Big should be changed to Endian.BIG in EmonModbusTcpInterfacer.py.