mailbox: zynq: add missing of_node_put before return

Fix following coccicheck warning:
WARNING: Function "for_each_available_child_of_node"
should have of_node_put() before return.

Early exits from for_each_available_child_of_node should decrement the
node reference counter.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>