[ALSA] CS4270 driver does not compile with I2C disabled

Fix compilation errors with the CS4270 when I2C is not enabled.  Updated
some comments to indicate that that stand-alone mode is not fully implemented,
because there is no mechanism for the CS4270 driver and the machine driver to
communicate the values of various input pins.
Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>