staging: greybus: make cport_quiesce() method optional

The cport_quiesce() method is mandatory in the case of
the es2 Greybus hd controller to shutdown the cports on
the es2 controller.
In order to add support of another controller which may not
need to shutdown its cports, make the cport_quiesce() optional,
and check if the controller implement it before to use it.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>