iwmc3200wifi: cfg80211 key hooks implemetation

This patch implements the new cfg80211 privacy related hooks: add/get/set_key
and the set_default_key one.
With this implementation we can now call the wext-compat *encode* routines and
reduce our own wext code.

Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>