staging: greybus: add host device function pointer checks

Add sanity checks for cport_quiesce and cport_clear before invoking the
callbacks as these function pointers are not required during the host
device registration. This follows the logic implemented elsewhere for
various other function pointers.

Signed-off-by: Jason Hrycay <jhrycay@gmail.com>
Reviewed-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>