Adding control elements is just for models supported by old
firewire-speakers modules. The processing should be in a function to add
model-dependent quirk.

This commit moves the codes to the function. As a result, the function
should handle error state, thus this commit also changes prototype of
the function.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>