cfg80211: fix the IE inheritance of extension IEs

Extension IEs have ID 255 followed by extension ID. Current
code is buggy in handling it in two ways:
1. When checking if IE is in the frame, it uses just the ID, which
   for extension elements is too broad.
2. It uses 0xFF to mark copied IEs, which will result in not copying
   extension IEs from the subelement.

Fix both issue.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>