Fix the check of the input-source type by checking the widget type of
each capture-source item.  Since some codecs can have both the mixer
and selector types depending on the ADC, alc_mux_enum_put() needs to
check each widget.

With this change, spec->capture_style gets unneeded, so it's removed,
too.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>