sys/oss/: advertise correct template caps - we indeed do non-native endianness and 8bit audio has no endianness

Original commit message from CVS:
* sys/oss/gstosssink.c:
* sys/oss/gstosssrc.c:
advertise correct template caps - we indeed do non-native endianness
and 8bit audio has no endianness
* sys/ximage/ximagesink.c: (gst_ximagesink_getcaps):
* sys/xvimage/xvimagesink.c: (gst_xvimagesink_getcaps):
avoid (wrong) duplications in getcaps function and return
template caps