cfg80211: remove WARN_ON() in cfg80211_sme_connect

cfg80211: remove WARN_ON() in cfg80211_sme_connect

A WARN_ON(wdev->conn) would trigger in cfg80211_sme_connect(), if multiple
send_msg(NL80211_CMD_CONNECT) system calls are made from the userland, which
should be anticipated and handled by the wireless driver. Remove this WARN_ON()
to prevent kernel panic if kernel is configured to "panic_on_warn".

Bug reported by syzbot.

Reported-by: syzbot+5f9392825de654244975@syzkaller.appspotmail.com
Signed-off-by: Du Cheng <ducheng2@gmail.com>
Link: https://lore.kernel.org/r/20210407162756.6101-1-ducheng2@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>