Get rid of some warnings

Get rid of some warnings

Mostly warnings about unused stuff.
Furthermore, the first hunk is a fix for the change in 177948a6.
Finally, comment in AEC_dtd was translated and the code simplified slightly.

CC     module_bluetooth_device_la-module-bluetooth-device.lo
modules/bluetooth/module-bluetooth-device.c: In function ‘a2dp_process_render’:
modules/bluetooth/module-bluetooth-device.c:1335:30: warning: pointer targets in passing argument 6 of ‘sbc_encode’
differ in signedness [-Wpointer-sign]
../src/modules/bluetooth/sbc/sbc.h:92:9: note: expected ‘ssize_t *’ but argument is of type ‘size_t *’

CC     module_rygel_media_server_la-module-rygel-media-server.lo
modules/module-rygel-media-server.c:383:13: warning: ‘append_property_dict_entry_object_array’ defined but not used [-Wunused-function]

CC     module_echo_cancel_la-adrian-aec.lo
modules/echo-cancel/adrian-aec.h:360:15: warning: ‘AEC_getambient’ defined but not used [-Wunused-function]
modules/echo-cancel/adrian-aec.h:368:14: warning: ‘AEC_setgain’ defined but not used [-Wunused-function]
modules/echo-cancel/adrian-aec.h:374:14: warning: ‘AEC_setaes’ defined but not used [-Wunused-function]
modules/echo-cancel/adrian-aec.h:377:16: warning: ‘AEC_max_dotp_xf_xf’ declared ‘static’ but never defined [-Wunused-function]

CC     module_echo_cancel_la-module-echo-cancel.lo
modules/echo-cancel/module-echo-cancel.c: In function ‘time_callback’:
modules/echo-cancel/module-echo-cancel.c:266:12: warning: variable ‘fs’ set but not used [-Wunused-but-set-variable]

CC     module-virtual-sink.lo
modules/module-virtual-sink.c: In function ‘sink_input_pop_cb’:
modules/module-virtual-sink.c:206:15: warning: variable ‘current_latency’ set but not used [-Wunused-but-set-variable]