Bluetooth: core: Fix missing power_on work cancel on HCI close

Move power_on work cancel to hci_dev_close_sync to ensure that power_on
work is canceled after HCI interface down, power off, rfkill, etc.

For example, if

    hciconfig hci0 down

is done early enough during boot, it may run before power_on work.
Then, power_on work will actually bring up interface despite above
hciconfig command.

Signed-off-by: Vasyl Vavrychuk <vasyl.vavrychuk@opensynergy.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>