pasemi_mac: Disable interface on close

Turns out we never disable the interface. It doesn't really cause
any problems since the channel is off, but it's still better to do it
this way.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>