common/fdt_support.c: avoid unintended return from fdt_fixup_memory_banks()

fdt_fixup_memory_banks() will add and update /memory node in
device tree blob. In the case that /memory node doesn't exist,
after adding a new one, this function returns error.

The correct behavior should be continuing to update its properties.

Signed-off-by: Miao Yan <miao.yan@windriver.com>