efi_loader: allow device == NULL in efi_dp_from_name()

This is a preparatory patch for use in efi_serialize_load_option()
as a load option's file_path should have both a device path and
a file path.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>