bluetooth/gst: Correct var type for GST_TYPE_BITMASK
GST_TYPE_BITMASK is 64-bit bit mask while corresponding channel_mask in pulseaudio is int therefore usually 32-bit. Switch to uint64_t instead to match internal representation in gstreamer.
Fixes pulseaudio crash on ARM 32-bit when pulseaudio is compiled with gstreamer and either LDAC or aptX support is available.