rndis_wlan: add missing padding to struct rndis_80211_remove_key

OID_802_11_REMOVE_KEY failed with invalid length error, add missing padding to
structure fix this.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>