iwm: Drain callouts after stopping the device during detach.

Otherwise there is a window where they may be rescheduled. This
typically manifested as a page fault shortly after unloading if_iwm.ko.
Close the race by draining callouts after calling iwm_stop_device(),
which is also what Dragonfly does.

Change whitespace to reduce gratuitous diffs with Dragonfly.

Reported and tested by: seanc
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation