mac80211: fix WMM TXOP calculation

In commit 9236c4523e5b ("mac80211: limit wmm params to comply
with ETSI requirements"), we have limited the WMM parameters to
comply with 802.11 and ETSI standard.  Mistakenly the TXOP value
was caluclated wrong.  Fix it by taking the minimum between
802.11 to ETSI to make sure we are not violating both.

Fixes: e552af05 ("mac80211: limit wmm params to comply with ETSI requirements")
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>