staging: bcm2835-audio: use anonymous union in struct vc_audio_msg

In this case explicitly naming the union doesn't help overall code
comprehension and clutters it.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>