802.11: rename Control Frame Extension field to match the spec

As reported by Richard Sharpe on -dev: https://www.wireshark.org/lists/wireshark-dev/201408/msg00236.html

Change-Id: I16638982e14e566bb7e584a2d030c546d2223ebe
Reviewed-on: https://code.wireshark.org/review/3916
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>