Keep power LED on during activity.

LED timer races with power LED disabling in hci_close(), resulting in
power LED left on after closing.
Stop LED timer before disabling power LED.

BTW. On cards without an activity LED, the behavior is a bit weird:
The LED is on after hci_open() but only until the first data transfer.
Then it's off in idle and on during activity.
It could be improved by keeping the LED on in idle and flashing during
activity.
Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>