cfg80211: fix set_regdom() to cancel requests with same alpha2

cfg80211: fix set_regdom() to cancel requests with same alpha2

While adding regulatory support to ath6kl I noticed that I easily
got the regulatory code confused. The way to reproduce the bug was:

1. iw reg set FI (in userspace)
2. cfg80211 calls ath6kl_reg_notify(FI)
3. ath6kl sets regdomain in firmware
4. firmware sends regdomain event to notify about the new regdomain (FI)
5. ath6kl calls regulatory_hint(FI)

And this (from FI to FI transition) confuses cfg80211 and after that I
only get "Pending regulatory request, waiting for it to be
processed...." messages and regdomain changes won't work anymore.

The reason why ath6kl calls regulatory_hint() is that firmware can change
the regulatory domain by it's own, for example due to 11d IEs. I could
of course workaround this in ath6kl but I think it's better to handle
the case in cfg80211.

The fix is pretty simple, use a different error code if the regdomain is
same and then just set the request processed so that it doesn't block new
requests.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>