greybus: svc: drop unnecessary 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).

The only remaining exception is gb_svc_intf_set_power_mode() which is
needed by the camera driver until proper link management is in place.

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