arch: riscv: fix logic error in parse_dtb

The function early_init_dt_scan returns true if a DTB was detected.

Fixes: 8fd6e05c

 ("arch: riscv: support kernel command line forcing when no DTB passed")
Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Tested-by: Paul Walmsley <paul.walmsley@sifive.com> # FU540 HiFive-U BBL
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>