cfg80211: Use the correct IE buffer pointer

If the IE buffer was allocated, the pub.information_elements pointer
was also changed to the allocated space. So we must not assume anymore
that the pointer points at the "found" tail.

So if it was allocated previously, take the codebranch that grows the
buffer size (if necessary) and put the data into the allocated buffer.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>