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.

Part-of: <pulseaudio/pulseaudio!723>
2 jobs for bitmask-type in 2 minutes and 3 seconds (queued for 1 second)