Update EIBConnection.rb

Monitoring the bus with EIBOpenVBusmonitorText, with some messages I received a negative value

__EIB_GetRequest()
__EIB_CheckRequest(block=true)
00000000  00 80                                            |..|
__EIB_CheckRequest received 2 bytes: )
__EIB_CheckRequest @head after recv. = [0, -128])