ALSA: hda - Distinguish each substream for better sticky assignment

ALSA: hda - Distinguish each substream for better sticky assignment

The commit a44caee6506b78aa4faa4d2ef567ece50c0d5980 introduced a
mechanism to assign the previously used slot for the next reopen of a
PCM stream.  But the PCM device number isn't always unique (it may
have multiple substreams), and also the code doesn't check the stream
direction, thus both playback and capture streams share the same
device number.

For avoiding this conflict, make a unique key for each substream and
store/check this value at reopening.

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