mtd: spi-nor: aspeed-smc: Add of_node_put()

Each iteration of for_each_available_child_of_node puts the previous
node, but in the case of a break from the middle of the loop, there is
no put, thus causing a memory leak. Upon termination of the loop
(whether by break or a natural exit), either ret will have a non-zero
value or child will be NULL. Hence add an of_node_put() that will
execute only when ret has a non-zero value, as calling of_node_put() on
a possible NULL value does not cause any further issues.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>