ath6kl: power down hardware when interface is down

The benefit from this is that user space can control hardware's power state
by putting interface up and down. This is handy if firmware gets to some
weird state.

The downside will be that putting interface up takes a bit longer,
I was measuring ~500 ms during interface up.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>