cfg80211: reg: remove warn_on for a normal case

If there are simulatenous queries of regdb, then there might be a case
where multiple queries can trigger request_firmware_no_wait and can have
parallel callbacks being executed asynchronously. In this scenario we
might hit the WARN_ON.

So remove the warn_on, as the code already handles multiple callbacks
gracefully.

Signed-off-by: Chaitanya Tata <chaitanya.tata@bluwireless.co.uk>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>