Had to change struct.pack in i2c_read_register for LinuxDevice to use @B instead of @b

Hi Martijn,

just wanted to let you know that I had to change the struct.pack in electronics.gateways.LinuxDevice to use unsigned bytes (@B, [as shown here](https://docs.python.org/3/library/struct.html#format-characters)). Couldn't get it to work with @b...