ALSA: usbmixer: rename usbmixer.[ch] -> mixer.[ch]

For clearer namespace, also rename usbmixer_maps.c -> mixer_maps.c

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/quirks.c

@@ -24,7 +24,7 @@
 
 #include "usbaudio.h"
 #include "card.h"
-#include "usbmixer.h"
+#include "mixer.h"
 #include "mixer_quirks.h"
 #include "midi.h"
 #include "quirks.h"