ASoC: BCM: Typo in RPi-DAC driver

This patch corrects a typo that originated from
inattentive copy and paste.

Signed-off-by: Florian Meier <florian.meier@koalo.de>
Merge pull request #656 from koalo/rpi-3.12.y

ASoC: BCM: Typo in RPi-DAC driver