codec_speex.c: Cleanup module loading to DECLINE and not FAILURE.

If codec_speex fails to register a translator it would cause Asterisk to
exit instead of continue as a DECLINED module.

* Make unload_module() always return 0.  It is silly to fail unloading if
any translators we try to unregister were not even registered.

Change-Id: Ia262591f68333dad17673ba7104d11c88096f51a