dpaa2-mac: Add a missing of_node_put after of_device_is_available

Add an 'of_node_put()' call when a tested device node is not available.

Fixes: 94ae899b

 ("dpaa2-mac: add PCS support through the Lynx module")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://lore.kernel.org/r/20201206151339.44306-1-christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>