Extract interface numbers into enums.

Bootloaders/CDC/Descriptors.c

Bootloaders/CDC/Descriptors.h

Bootloaders/DFU/BootloaderDFU.h

Bootloaders/DFU/Descriptors.c

Bootloaders/DFU/Descriptors.h

Bootloaders/HID/Descriptors.c

Bootloaders/HID/Descriptors.h

Bootloaders/MassStorage/BootloaderMassStorage.c

Bootloaders/MassStorage/Descriptors.c

Bootloaders/MassStorage/Descriptors.h

Bootloaders/Printer/BootloaderPrinter.c

Bootloaders/Printer/Descriptors.c

Bootloaders/Printer/Descriptors.h

Demos/Device/ClassDriver/AudioInput/AudioInput.c

Demos/Device/ClassDriver/AudioInput/Descriptors.c

Demos/Device/ClassDriver/AudioInput/Descriptors.h

Demos/Device/ClassDriver/AudioOutput/AudioOutput.c

Demos/Device/ClassDriver/AudioOutput/Descriptors.c

Demos/Device/ClassDriver/AudioOutput/Descriptors.h

Demos/Device/ClassDriver/DualVirtualSerial/Descriptors.c