sys/oss4/: Make device-name probing in NULL state work better (e.g. for the gnome-control-center sound capplet).

Original commit message from CVS:
* sys/oss4/oss4-mixer.c: (gst_oss4_mixer_open):
* sys/oss4/oss4-property-probe.c:
(gst_oss4_property_probe_find_device_name),
(gst_oss4_property_probe_find_device_name_nofd):
* sys/oss4/oss4-property-probe.h:
* sys/oss4/oss4-sink.c: (gst_oss4_sink_get_property):
* sys/oss4/oss4-source.c: (gst_oss4_source_get_property):
Make device-name probing in NULL state work better (e.g. for the
gnome-control-center sound capplet).