libqiprog: Move qiprog_address struct to private header

The qiprog_address struct was really designed to carry data over USB, and
does not belong in the public API. Move it to qiprog_internal.h

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
doc: Document public QiProg structures and enumerators

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>