ucm: add existence checks to geti calls

ucm: add existence checks to geti calls

Right now in snd_use_case_geti you cannot tell if the item being queried
exists or not when being checked. This also means the only way to check
for the existence of something in the client of the library is to
iterate over the list of mods/devs even if we know exactly the name we
are looking for. We have functions that do exactly this internally so
lets return this information in a logical fashion through geti.

Also clean up some trailing white space nearby.

Signed-off-by: Curtis Malainey <cujomalainey@chromium.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>