My previous commit (b3ef4013a4) added a couple of lock assertions.
However, the detach assertion is wrong - if the driver hasn't
attached then detach needs to be a no-op and this includes
not touching a deinit'ed mutex.
So, move the mutex check and use until after the attached check.
This fixes the driver crash I saw when another bit of in-flight
work caused attach to fail, and thus iwm_detach() -> iwm_detach_local()
ended up trying to use a deinit'ed mutex, leading to a crash.