ALSA: hda - Don't export symbols when built-in kernel

The global functions in hda_codec.c and other core parts are only
for HD-audio codec and controller drivers.  When the HD-audio driver
is built in kernel, all stuff have to be statically linked, thus
we don't need any exports.

This patch introduces a conditional macro to do export only
when needed.

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