Only set SER_RS485_ENABLED bit of existing RS485 settings
Only set SER_RS485_ENABLED bit of existing RS485 settings

Thanks to @JCWren

stephane/libmodbus,marinbek/libmodbus,OliverDDS/libmodbus,marinbek/libmodbus,stephane/libmodbus,marinbek/libmodbus,stephane/libmodbus,OliverDDS/libmodbus,OliverDDS/libmodbus