Take Riser card MUX address from the command line

We have a setup FELIX server with Supermicro X11SRA-RF motherboard. While we don't have a Riser card, we can still communicate to the FLX712 I2C via the SMBus. There is a similar MUX on the motherboard at 0x71, but the Riser card MUX address is hard-coded to 0x70 in fflash tool. I've added a command line parameter to set a custom "Riser card MUX" address, so that fflash can be used with our setup.