staging: vc04_services: Fix wrong early return in next_service_by_instance()

If kref_get_unless_zero() fails, we should keep looking for the
next service, since the callers of this function expect that a NULL
return value means there are no more.

Signed-off-by: Marcelo Diop-Gonzalez <marcgonzalez@google.com>
Link: https://lore.kernel.org/r/20200213194001.130110-1-marcgonzalez@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>