Documentation: fix DOCBOOKS=... building

Documentation: fix DOCBOOKS=... building

Prior to commit 4266129964b8 ("[media] DocBook: Move all media docbook
stuff into its own directory") it was possible to build only a single
(or more) book(s) by calling, for example

    make htmldocs DOCBOOKS=80211.xml

This now fails:

    cp: target `.../Documentation/DocBook//media_api' is not a directory

Ignore errors from that copy to make this possible again.

Fixes: 4266129964b8 ("[media] DocBook: Move all media docbook stuff into its own directory")
Signed-off-by: Johannes Berg <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>