ALSA: hda - Make CONFIG_SND_HDA_CODEC_* tristate

So far, CONFIG_SND_HDA_CODEC_* kconfigs have been booleans due to
historical reasons.  The major reason was that the automatic codec
driver probing wouldn't work if user sets a codec driver as a module
while the controller driver as a built-in.  And, another reason was to
avoid exporting symbols of the helper codes when all drivers are built
in.

But, this sort of "kindness" rather confuses people in the end,
especially makes the config refinement via localmodconfig unhappy.
Also, a codec module would still work if you re-bind the controller
driver via sysfs (although it's no automatic loading), so there might
be a slight use case.

That said, better to let people fallen into a pitfall than being too
smart and restrict something.  Let's make things straightforward: now
all CONFIG_SND_HDA_CODEC_* become tristate, and all symbols exported
unconditionally.

Signed-off-by: Takashi Iwai <tiwai@suse.de>