spi: cadence: Add of_node_put() before return

Fix following coccicheck warning:
./drivers/spi/spi-cadence-xspi.c:490:1-23: WARNING: Function
for_each_child_of_node should have of_node_put() before return

Early exits from for_each_child_of_node should decrement the
node reference counter.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Link: https://lore.kernel.org/r/20211015033919.5915-1-wanjiabing@vivo.com
Signed-off-by: Mark Brown <broonie@kernel.org>