This patch does two things:
- code refactoring with a local helper function,
- allow codec drivers to provide the specific PCM stream info pointers
  only for overriding the non-NULL entries, instead of copying the
  whole.

This simplifies the codec driver side (currently the only user is
alc269's 44kHz fixed rate).
Signed-off-by: Takashi Iwai <tiwai@suse.de>