cmd_fdt: fix working_fdt is set to wrong value

Instead of setting working_fdt to map_sysmem(addr) (e.g. blob), it should be set
to addr directly as inside set_working_fdt_addr it uses map_sysmem(addr) again.
To test: ./u-boot -d dts/dt.bin , then issue: fdt addr 0x100, fdt print will
then cause an segmentation fault. After this fix fdt print is functional.