ALSA: hda - Fix inconsistent input_paths after ADC reduction

In the current parser code, the input_paths[] may become inconsistent
when some of detected ADCs are dropped due to incomplete inputs, since
the driver rearranges only adc_nids[] but doesn't touch input_paths[].

This patch fixes the issue, and also it optimizes the reachability
checks by simply referring to the parsed input_paths[] instead of
calling is_reachable() again for each connection.

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