mac80211_hwsim: correctly register the platform driver

Not registering a platform_driver would make us access garbage
when the platform callbacks under driver_register() kicks in.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Tested-By: Martin Pitt <martin.pitt@ubuntu.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>