riscv/efi_stub: Fix get_boot_hartid_from_fdt() return value

The get_boot_hartid_from_fdt() function currently returns U32_MAX
for failure case which is not correct because U32_MAX is a valid
hartid value. This patch fixes the issue by returning error code.

Cc: <stable@vger.kernel.org>
Fixes: d7071743 ("RISC-V: Add EFI stub support.")
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>