memory: stm32-fmc2-ebi: add missing of_node_put for loop iteration

Early exits from for_each_available_child_of_node() should decrement the
node reference counter.  Reported by Coccinelle:

  drivers/memory/stm32-fmc2-ebi.c:1046:1-33: WARNING:
    Function "for_each_available_child_of_node" should have of_node_put() before return around line 1051.

Fixes: 66b8173a ("memory: stm32-fmc2-ebi: add STM32 FMC2 EBI controller driver")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Christophe Kerello <christophe.kerello@foss.st.com>
Link: https://lore.kernel.org/r/20210423101815.119341-1-krzysztof.kozlowski@canonical.com