mac80211_hwsim: Register and bind to driver

Properly register our mac80211_hwsim_driver, attach it to the platform bus.
Bind newly created hwsim devices to that driver, so that our wlan devices get
a proper "driver" sysfs attribute.

This makes mac80211_hwsim interfaces work with NetworkManager.

Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>
[fix an old and a new message to not be line-broken,
 also fix the driver_register error path]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>