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>
diff --git a/sound/usb/card.c b/sound/usb/card.c
index 78d12ff..0bd62a1 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -56,7 +56,7 @@
 #include "usbaudio.h"
 #include "card.h"
 #include "midi.h"
-#include "usbmixer.h"
+#include "mixer.h"
 #include "proc.h"
 #include "quirks.h"
 #include "endpoint.h"