mwl8k: fix condition in mwl8k_cmd_encryption_remove_key()

The intent here was to check whether key->cipher was WEP40 or WEP104.
We do a similar check correctly in several other places in this file.
The current condition is always true.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>