ALSA: vmaster: Fix slave change notification

When a value of a vmaster slave control is changed, the ctl change
notification is sometimes ignored.  This happens when the master
control overrides, e.g. when the corresponding master control is
muted.  The reason is that slave_put() returns the value of the actual
slave put callback, and it doesn't reflect the virtual slave value
change.

This patch fixes the function just to return 1 whenever a slave value
is changed.

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