wl1271: Fix WEP key handling

WEP key index handling was broken: the default key when using key 0 was never
specified to the FW, and if using other default than 0, it would be set on the
TX path for every single TX'd frame.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>