nl80211: add support for mcs masks

Allow to set mcs masks through nl80211. We also allow to set MCS
rates but no legacy rates (and vice versa).

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>