cfg80211: Define nla_policy for NL80211_ATTR_LOCAL_MESH_POWER_MODE

Buffer overread may happen as nl80211_set_station() reads 4 bytes
from the attribute NL80211_ATTR_LOCAL_MESH_POWER_MODE without
validating the size of data received when userspace sends less
than 4 bytes of data with NL80211_ATTR_LOCAL_MESH_POWER_MODE.
Define nla_policy for NL80211_ATTR_LOCAL_MESH_POWER_MODE to avoid
the buffer overread.

Fixes: 3b1c5a5307f ("{cfg,nl}80211: mesh power mode primitives and userspace access")
Cc: stable@vger.kernel.org
Signed-off-by: Srinivas Dasari <dasaris@qti.qualcomm.com>
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
