nl80211: Fix NL80211_ATTR_SMPS_MODE encoding

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>