mac80211: don't warn about CW params when not using them

ieee80211_set_wmm_default() normally sets up the initial CW min/max for
each queue, except that it skips doing this if the driver doesn't
support ->conf_tx. We still end up calling drv_conf_tx() in some cases
(e.g., ieee80211_reconfig()), which also still won't do anything
useful...except it complains here about the invalid CW parameters.

Let's just skip the WARN if we weren't going to do anything useful with
the parameters.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Link: https://lore.kernel.org/r/20190718015712.197499-1-briannorris@chromium.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>