mac802154: allow only one WPAN to be up at any given time

All 802.15.4 PHY devices with drivers in tree can support only one WPAN
at any given time, yet the stack allows arbitrarily many WPAN devices to
be created and up at the same time. This cannot work with what the
hardware provides, and in the current implementation, provides an easy
DoS vector to any process on the system that may call socket() and
sendmsg().

Thus, allow only one WPAN per PHY to be up at once, just like mac80211
does for managed devices.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>