[ALSA] es1688 - code clean-up

Seperate out the legacy probing into its own function, improving
readability.

Signed-off-by: Rene Herman <rene.herman@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>