sys/oss4/oss4-mixer-slider.c: Apparently mono sliders have the mono value repeated in the upper bits, so mask those o...

Original commit message from CVS:
Based on patch by: Clive Wright <clive_wright ntlworld com>
* sys/oss4/oss4-mixer-slider.c: (gst_oss4_mixer_slider_unpack_volume):
Apparently mono sliders have the mono value repeated in the upper bits,
so mask those out when reading them. Probably makes the mixer applet
work properly in some more cases.