docbook: use IDs as filenames to support multiple books

I'm trying to generate a <set> of <book>s in docbook for wireless to
link together all the cfg80211 and mac80211 documentation.

However, docbook will generate "re01.html" anew for each book for the
first <refentry>, presumably due to a bug in the stylesheets.

An effective workaround is to use IDs for the filenames, which makes
them more descriptive as well, e.g.  API-enum-ieee80211-band.html.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>