Documentation: fix DOCBOOKS=... building

Prior to commit 42661299 ("[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: 42661299

 ("[media] DocBook: Move all media docbook stuff into its own directory")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>