mac80211_hwsim: Set wmediumd for new radios

Set the wmediumd to the net's wmediumd when the radio gets created.
Radios created after HWSIM_CMD_REGISTER don't currently get their
data->wmediumd set and the userspace would need to reconnect to
netlink to be able to call HWSIM_CMD_REGISTER again.

Alternatively I think data->netgroup and data->wmedium could be
replaced with a pointer to hwsim_net.

Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>