[ALSA] fix private data pointer calculation in CS4270 driver

Fix the calculation of the private_data pointer in the CS4270 driver.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>