platform/chrome: cros_ec_typec: Fix remove partner logic

The cros_unregister_ports() function can be called in situations where
the partner has not been registered yet, and so its related data
structures would not have been initialized. Calling
cros_typec_remove_partner() in such a situation can lead to null pointer
dereferences. So, only call cros_typec_remove_partner() if there is a
valid registered partner pointer.

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20201029222738.482366-3-pmalani@chromium.org