iwx: fix kernel panic on module unload with assertions enabled

[avos]

Summary

When interface is up and running 'kldunload if_iwx' stops the device and executes RUN -> INIT state transition.
Since the device is already stopped iwx_run_stop fails to stop the device again and returns non-zero exit code from iv_newstate callback which triggers 'INIT state change failed' assertion.

I reused IWX_FLAG_SHUTDOWN flag to:
a) set it in iwx_detach
b) check it in iwx_newstate_sub - when it is set all custom state transition logic is skipped

Accidentally found while experimenting with iwlwifi / iwx drivers

Test Plan

Tested with Intel AX211, STA mode

[avos]

Entirely forgot about locking

[adrian]

oh i hit this today! good catch.

[avos]

@adrian may I ask to commit this change? I'm still catching up with changes and did not ask to restore write access yet.

[adrian]

of course! i'll do it shortly.