mwl8k: Fix broken WEP

The WEP key length was being set to 0 erroneously which broke WEP support.
Fix the same by setting the key length appropriately.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>