cfg80211: make wiphy index start at 0 again

The change to use atomic_inc_return() for assigning the wiphy
index made the first wiphy index 1 instead of 0. This is fine,
but we all habitually type "phy0" when we're testing, so make
it go back to 0 instead of 1 by subtracting 1 from the index.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>