of/fdt: fix error checking for earlycon address

fdt_translate_address() returns OF_BAD_ADDR on error.  It is defined as
a u64 value, so the variable "addr" should be defined as u64 as well.

Fixes: fb11ffe7 ("of/fdt: add FDT serial scanning for earlycon")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>