efi_loader: correct check in efi_load_option_dp_join()

After assigning to *dp we must check this value and not dp.

Addresses-Coverity-ID: 501792 Control flow issues (DEADCODE)
Fixes: 58bef195f94e ("cmd: eficonfig: add support for setting fdt")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
1 file changed