cfg80211: invert P2P-Device vs. netdev check ordering

In cfg80211_can_use_iftype_chan(), check for P2P Device
first, and then for netdevs. This doesn't really change
anything but makes the code a bit easier to read since
it may not be obvious for everyone at first that a P2P
device has no netdev.

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