ALSA: include/uapi/sound/firewire.h: use "_UAPI" instead of "UAPI"

When installing, "scripts/headers_install.sh" will strip guard macro'
"_UAPI" to prevent from appearing it to users. And also, all another
files which need uapi prefix always use "_UAPI", not "UAPI".

So use "_UAPI" instead of "UAPI" on the guard macro, and also give a
comment for "#endif".

Signed-off-by: Chen Gang <[e-mail]>
Acked-by: Clemens Ladisch <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

include/uapi/sound/firewire.h

···
-#ifndef UAPI_SOUND_FIREWIRE_H_INCLUDED
-#define UAPI_SOUND_FIREWIRE_H_INCLUDED
+#ifndef _UAPI_SOUND_FIREWIRE_H_INCLUDED
+#define _UAPI_SOUND_FIREWIRE_H_INCLUDED
 
 #include <linux/ioctl.h>
 
···
  * Returns -EBUSY if the driver is already streaming.
  */
 
-#endif
+#endif /* _UAPI_SOUND_FIREWIRE_H_INCLUDED */