libfdt: fix fdt_stringlist_search()

If fdt_getprop() fails, negative error code should be returned.

[ DTC commit: daa75e8fa5942caa8e97931aed3a1ee0b7edd74b ]

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>