mac80211: add p2p device type support

When a driver advertises p2p device support,
mac80211 will handle it, but internally it will
rewrite the interface type to STA/AP rather than
P2P-STA/GO since otherwise a lot of paths need
to be touched that are otherwise identical. A
p2p boolean tells drivers whether or not a given
interface will be used for p2p or not.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>