dpaa2-switch: check if the port priv is valid

Before accessing the port private structure make sure that there is
still a non-NULL pointer there. A NULL pointer access can happen when we
are on the remove path, some switch ports are unregistered and some are
in the process of unregistering.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>