nl80211: Fix nla_put_u8 to u16 for NL80211_WMMR_TXOP

TXOP (also known as Channel Occupancy Time) is u16 and should be
added using nla_put_u16 instead of u8, fix that.

Fixes: 50f32718 ("nl80211: Add wmm rule attribute to NL80211_CMD_GET_WIPHY dump command")
Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>