ucm: fix some variable constness issues

ucm: fix some variable constness issues

I submitted earlier a patch that made the value parameter of
snd_use_case_get() non-const, but as that changed the public API, the
patch couldn't be accepted. This is the same patch, modifying the
internal code so that there are fewer issues with constness, but the
public API is left alone (a comment was added to the function
documentation, though, so that hopefully nobody else will try to fix
the same unfixable problem).

Signed-off-by: Tanu Kaskinen <tanu.kaskinen@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>