[ALSA] Fix CS4270 volume control and optimize I2C operations

The volume control for the CS4270 ASoC driver was inverted - raising the
volume level with alsamixer would decrease the actual volume.
This patch also improves the performance of the I2C code (used to change
register settings) by only performing an I2C write if the new value is
different from the value that's in the register cache.

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