ALSA: hda - Use "Capture Source" for single sources

In general we prefer "Capture Source" to "Input Source".
The latter was chosen in many places just because "Capture Source"
label doesn't work well with the current alsa-lib mixer abstraction
when multiple instances are present.  But when we know that there is a
single input-source element, we can safely choose "Capture Source"
label.

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