Fix compiler warnings

Fixes these warnings:

```
USBLamp.cpp: In member function ‘void USBLamp::setColor(Color)’:
USBLamp.cpp:98:84: warning: narrowing conversion of ‘((USBLamp*)this)->USBLamp::color.Color::red’ from ‘unsigned char’ to ‘char’ inside { } is ill-formed in C++11 [-Wnarrowing]
     char data[] = {color.red, color.green, color.blue, 0x00, 0x00, 0x00, 0x00, 0x05};
                                                                                    ^
USBLamp.cpp:98:84: warning: narrowing conversion of ‘((USBLamp*)this)->USBLamp::color.Color::green’ from ‘unsigned char’ to ‘char’ inside { } is ill-formed in C++11 [-Wnarrowing]
USBLamp.cpp:98:84: warning: narrowing conversion of ‘((USBLamp*)this)->USBLamp::color.Color::blue’ from ‘unsigned char’ to ‘char’ inside { } is ill-formed in C++11 [-Wnarrowing]
```