UCM: Disallow gets of undefined system properties

In UCM, there are various system-defined properties whose names start
with "_". Explicitly prevent any gets from falling back to properties
defined in a config file if the property name starts with "_", in order
to reserve the entire "_" namespace for system-defined properties.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>