boot_fit: Change return value from FDT_ERROR to -EINVAL in fdt_offset()

boot_fit: Change return value from FDT_ERROR to -EINVAL in fdt_offset()

FDT_ERROR is defined as unsigned long. However, since the return value of
fdt_offset() is int, a warning will occur when compiling. Also, it is better
to use -EINVAL than FDT_ERROR.
This fixes this problem by change return value from FDT_ERROR to -EINVAL.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
CC: Franklin S Cooper Jr <fcooper@ti.com>
Reviewed-by: Franklin S Cooper Jr <fcooper@ti.com>