spi: fsi: add a missing of_node_put

'for_each_available_child_of_node' performs an of_node_get on each
iteration, so a return from the middle of the loop requires an of_node_put.

Fixes: bbb6b2f9 ("spi: Add FSI-attached SPI controller driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/504e431b900341249d331b868d90312cf41f415a.1618947919.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Mark Brown <broonie@kernel.org>