staging: bcm2835-audio: Use card->private_data

Instead of allocating a separate snd_device object, let snd_card_new()
allocate the private resource.  This simplifies the code.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>