cfg80211: regulatory: make initialization more robust

Since my change to split out the regulatory init to occur later,
any issues during earlier cfg80211_init() or errors during the
platform device allocation would lead to crashes later. Make this
more robust by checking that the earlier initialization succeeded.

Fixes: d7be102f ("cfg80211: initialize regulatory keys/database later")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>