cfg80211: expose the rfkill device to the low level driver

This will allow the low level driver to query the rfkill
state.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Link: https://lore.kernel.org/r/20210616202826.9833-1-emmanuel.grumbach@intel.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>