ALSA: hda - Introduce snd_hda_get_pin_label()

Create a new helper function snd_hda_get_pin_label() for getting a label
string for both input and output pins.  hda_get_input_pin_label() is
obsoleted by this function, and the callers are replaced appropriately
now by this patch.

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