mac80211: initialize sta pointer to avoid false-positive warning

Some compilers (eg. gcc 4.4.1 for ARM) report a false positive warning
in mlme.c:

net/mac80211/mlme.c: In function 'ieee80211_prep_connection':
net/mac80211/mlme.c:3035: warning: 'sta' may be used uninitialized in this function

This is a false positive because the place where 'sta' is used is
inside an if with the same condition of where it is set:

[...]
        if (!have_sta) {
                sta = sta_info_alloc(sdata, cbss->bssid, GFP_KERNEL);
                if (!sta)
                        return -ENOMEM;
        }
[...]
        if (!have_sta) {
[...]
                sta->sta.supp_rates[cbss->channel->band] = rates;
[...]

For some reason the compiler doesn't understand this and warns.

While this is not a problem in the code itself, we can avoid polluting
the build logs with false positives by setting sta to NULL on
declaration and checking for sta instead of !have_sta in the second if.

Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>