rt2x00: error in configurations with mesh support disabled

If CONFIG_MAC80211_MESH is not set, cfg80211 will now allow advertising
interface combinations with NL80211_IFTYPE_MESH_POINT present.
Add appropriate ifdefs to avoid running into errors.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>