In iwn(4), delay 40MHz RXON config until we have negotiated 11n mode.

In iwn(4), delay 40MHz RXON config until we have negotiated 11n mode.

Enabling 40MHz too early can cause firmware sysassert 0x05 on 5300 devices
when we send the initial AUTH frame to kick off association. At this point
we haven't negotiated 11n mode yet (this happens later during the association
sequence), so perhaps firmware saw an inconsistent configuration state?
It is unclear why this problem did not seem to affect any other devices.

Reported by Christian Schulte on misc@