usb/gadget/{f_audio,gmidi}.c: follow recent changes in audio.h

Some structs in linux/usb/audio.h have got new names to mark them as
part of version 1.0 of the USB audio standard. Follow these changes
in the gadget drivers.

Note that this header and the ALSA USB driver will undergo some
refactoring soon, so there might be another update to the gadgets as
well.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>