ALSA: usb-audio: simplify control interface access

As the control interface is now carried in struct snd_usb_audio, we can
simplify the API a little and also drop the private ctrlif field from
struct usb_mixer_interface.

Also remove a left-over function prototype in pcm.h.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/clock.h b/sound/usb/clock.h
index beb2536..4663093 100644
--- a/sound/usb/clock.h
+++ b/sound/usb/clock.h
@@ -5,8 +5,6 @@
 			     struct usb_host_interface *alts,
 			     struct audioformat *fmt, int rate);
 
-int snd_usb_clock_find_source(struct snd_usb_audio *chip,
-			      struct usb_host_interface *host_iface,
-			      int entity_id);
+int snd_usb_clock_find_source(struct snd_usb_audio *chip, int entity_id);
 
 #endif /* __USBAUDIO_CLOCK_H */