greybus: svc: drop all symbol exports

The svc functions are only supposed to be called by core and should not
be exported.

Most of these functions should never have been exported in the first
place, while a few no longer needs to be (e.g. since core gained support
for offloaded connections).

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>