ALSA: hda/realtek - Don't create alt-stream for capture when unnecessary

When the driver finds multiple ADCs, it tries to create an alternative
capture PCM stream.  However, these secondary ADCs might be useless or
in uncontrolled paths in some cases, e.g. when auto-mic or dynamic
ADC-switching is enabled.  Also, when only a single capture source is
available, the multi-streams don't make sense, too.

With this patch, the driver checks such condition and skips the alt
stream appropriately.

Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>