rndis_wlan: use RNDIS_WLAN_NUM_KEYS for all key index checks

Use new RNDIS_WLAN_NUM_KEYS for checks in add_wep_key() and add_wpa_key().

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