nl80211: Fix NL80211_ATTR_SMPS_MODE encoding

This nl80211 attribute uses NLA_U8 policy in cfg80211 and
hostapd/wpa_supplicant needs to use same size when writing the
attribute.

This fixes AP mode regression triggered by kernel commit "net: netlink:
Update attr validation to require exact length for some types" in
v4.15-rc1 that resulted in the following debug log entry when trying to
enable beaconing:
nl80211: Beacon set failed: -34 (Numerical result out of range)

Fixes: da1080d7215f ("nl80211: Advertise and configure SMPS modes")
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>

src/drivers/driver_nl80211.c

@@ -3957,7 +3957,7 @@ static int wpa_driver_nl80211_set_ap(void *priv,
 			smps_mode = NL80211_SMPS_OFF;
 			break;
 		}
-		if (nla_put_u32(msg, NL80211_ATTR_SMPS_MODE, smps_mode))
+		if (nla_put_u8(msg, NL80211_ATTR_SMPS_MODE, smps_mode))
 			goto fail;
 	}