Make sure iwn(4) firmware and driver stay in sync when a scan is

Make sure iwn(4) firmware and driver stay in sync when a scan is
requested by ifconfig while associated. For completeness, do the same
for RUN->{ASSOC,AUTH} and AUTH->ASSOC transitions. i.e. always keep
the firmware's association state in sync with the driver's state.
The firmware should only be associated in RUN state.

Fixes a problem where the driver remained in SCAN state forever after
running 'ifconfig iwn0 scan' in associated (i.e. RUN) state, presumably
because the firmware didn't like the driver's scan command and never
signaled completion of the scan.

ok kevlo@ phessler@