libertas: Reduce the WPA key installation time.

libertas: Reduce the WPA key installation time.

WPA requires that the PTK is installed immediately after the 4-way handshake
in order to properly decrypt the subsequent incoming EAPOL-GTK frame.  If the
PTK is not enabled by the time the EAPOL-GTK frame arrives, the frame is
dropped and the supplicant does not receive the group key.

This will happen with fast Access Points that send the EAPOL-GTK frame before
the suplicant has successfully installed and enabled the PTK.  To mitigate
this situation, this patch simplifies and accelerates the SIOCSIWENCODEEXT
execution.

This patch resolves OLPC ticket 7825 (http://dev.laptop.org/ticket/7825)

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>