ALSA: add snd_ctl_add_slave_uncached()

Added snd_ctl_add_slave_uncached() function to add a slave element
with volatile controls.  The values of normal slave elements are
supposed to be cachable, i.e. they are changed only via the put
callbacks.  OTOH, when a slave element is volatile and its values may
be changed by other reason (e.g. hardware status change), the values
will get inconsistent.

The new function allows the slave elements with volatile changes.
When the slave is tied with this call, the native get callback is
issued at each time so that the values are always updated.

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