dpaa2-eth: check fsl_mc_get_endpoint for IS_ERR_OR_NULL()

The fsl_mc_get_endpoint() function can return an error or directly a
NULL pointer in case the peer device is not under the root DPRC
container. Treat this case also, otherwise it would lead to a NULL
pointer when trying to access the peer fsl_mc_device.

Fixes: 71947923 ("dpaa2-eth: add MAC/PHY support through phylink")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>