This stops the panics that occur on MIPS platforms when doing say,
'sysctl dev.ath.0' whilst the MAC is asleep. The MIPS platform is
rather unforgiving in getting power-save register access wrong and you
will get all kinds of odd failures if you don't have things woken
up at the right times.
Tested:
* QCA9558 (TP-Link Archer C7 v2)
* AR9331 (Carambola 2)
.. with no VAPs configured and ath0 down (thus the MAC is definitely
asleep.)
PR: kern/201117
52f5515397