cfg80211: update comment about WIPHY_FLAG_CUSTOM_REGULATORY

Commit a2f73b6c ("cfg80211: move regulatory flags to their own variable")
renamed WIPHY_FLAG_CUSTOM_REGULATORY to REGULATORY_CUSTOM_REG, but missed to
update one comment.

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