Add missing function attributes.

LUFA/Drivers/Peripheral/AVR8/SPI_AVR8.h

LUFA/Drivers/USB/Class/Host/Audio.h

LUFA/Drivers/USB/Class/Host/CDC.h

LUFA/Drivers/USB/Class/Host/HID.c

LUFA/Drivers/USB/Class/Host/HID.h

LUFA/Drivers/USB/Class/Host/MIDI.h

LUFA/Drivers/USB/Class/Host/MassStorage.h

LUFA/Drivers/USB/Class/Host/Printer.h

LUFA/Drivers/USB/Class/Host/RNDIS.h

LUFA/Drivers/USB/Class/Host/StillImage.h

LUFA/Drivers/USB/Core/AVR8/Device_AVR8.h

LUFA/Drivers/USB/Core/AVR8/Pipe_AVR8.h

LUFA/Drivers/USB/Core/HostStandardReq.h

LUFA/Drivers/USB/Core/StdRequestType.h

LUFA/Drivers/USB/Core/UC3/Device_UC3.h

LUFA/Drivers/USB/Core/UC3/Pipe_UC3.h