ihidev(4): Fix locking and interrupt handler.

- Can't run iic_exec in softint because it does cv_wait, at least on
  some i2c controllers -- defer to workqueue instead.

- Fix violations of locking rules:
  . Do not take a lock at higher IPL than it is defined at!
  . Do not sleep under a lock!
  . Definitely do not sleep under a spin lock!
  In this case, sc_intr_lock was defined at IPL_VM but used at IPL_TTY,
  and i2c transactions -- possibly causing sleep for cv_wait -- were
  issued under it.

  But in this case, the interrupt handler needs only a single bit to
  mark whether the work is pending, so just use atomic_swap for that.

- Use an adaptive lock (IPL_NONE) for i2c transactions.

- Detach children, and do so before freeing anything.

(riastradh)