audioclock: use GstAudioClock* as first argument in GstAudioClock methods

audioclock: use GstAudioClock* as first argument in GstAudioClock methods

All the GstAudioClock method declarations required object of GstClock type
as a first argument, but in fact, required GstAudioClock object (runtime
check in function body). Instead of checking type in run-time, we can
change functions declaration, to accept only GstAudioClock methods. Then,
runtime check is not necessary anymore, since always GstAudioClock object
is passed to a function.

https://bugzilla.gnome.org/show_bug.cgi?id=756628