mwl8787: dont set IEEE80211_CONF_PS for mesh ps

This flag is meant for managed STA ps mode, and it changes meanings
depending on what the driver implements.  Most drivers we use will
go to sleep as soon as IEEE80211_CONF_PS bit is set, and consequently
they will ignore any mesh traffic (whereas with mesh PS we want to
put them to sleep only after we get their power saving state from
a beacon).

Signed-off-by: Bob Copeland <me@bobcopeland.com>