staging: bcm2835-audio: Simplify card object management

Instead of creating a dummy child device to manage the card object,
just use devm stuff directly for releasing with snd_card_free().
This results in a lot of code reduction.

Since the dummy child devices are gone, the device object to be passed
to the memory allocator needs to be adjusted as well.

Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>