rndis_wlan: add range check in del_key()

Wifi drivers can have up to 6 keys but the rndis_wlan only has 4 so
it needs to have its own checks to make sure we don't go out of
bounds.  The add_key() function already checks but I added some
checks to del_key() and set_default_key().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>