ALSA: sound/spi: patch for the unuseful variable removal

The '*bitclk' of structure 'snd_at73c213' seems no use,
so I make a patch to remove the unnecessary variable.

Signed-off-by: Wan ZongShun <mcuos.com@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>