iwm: improve rfkill handling

Previously the driver handled the bit within itself, but did not expose
the state change to net80211 and interface layers.
This change uses net80211 KPI for rfkill signaling.
The code is modeled after similar code in iwn and wpi.

Reviewed by: adrian
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24923