I didn't check the specs, but at least experience with real-world cards
(and modems) shows that it's not just permitted to select the immediate
parent DF, but all ancestors of the currently selected file.
So adjust the get_selectables() method to not just return the immediate
parent, but to recurse all the way up and report the FID of any ancestor
DF.
Change-Id: Ic9037aa9a13af6fb0c2c22b673aa4afa78575b49