loader: edd_device_path_v3 is too small

The EDD v3[1], see table 13, page 33, does define device path as double
qword, that is, 16 bytes, we have only qword.

Also remove edd_device_path_v4 and edd_params_v4 because those are not used,
and there is no size difference in v3 versus v4.

[1] http://www.t13.org/documents/UploadedDocuments/docs2004/d1572r3-EDD3.pdf

MFC after: 2 weeks