ALSA: ca0106 Add comments to snd_ca0106_details struct

Takashi wrote an email [1] explaining the fields of snd_ca0106_details,
so I captured the information into the ca0106.h header file.

[1] http://article.gmane.org/gmane.linux.alsa.devel/56783/match=takashi+gpio_type



Signed-off-by: Ben Stanley <Ben.Stanley@exemail.com.au>
Signed-off-by: Takashi Iwai <tiwai@suse.de>