wifi: mac80211: add a helper to fragment an element

wifi: mac80211: add a helper to fragment an element

The way this works is that you add all the element data,
keeping a pointer to the length field of the element.
Then call this helper function, which will fragment the
element if there was more than 255 bytes in the element,
memmove()ing the data back if needed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>