[PATCH] Add return value to indicate unsopported cipher

Hello. Right now there's no way for user of osmo_a5(..) to understand if particular cipher (a5/3 for example) is supported or not. Attached patch adds simple return value to osmo_a5 to fix that. Of course I would rather see a5/3 included into mainline but in a meantime this patch might be useful too :) -- best regards, Max, http://fairwaves.ru