fdt_support: correct the return condition of fdt_initrd()

Before this commit, fdt_initrd() just returned if initrd
start address is zero.
But it is possible if the RAM is located at address 0.

This commit makes the return condition more reasonable:
Just return if the size of initrd is zero.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>