Bluetooth: close HCI device when user channel socket gets closed

With 9380f9ea the order of unsetting
the HCI_USER_CHANNEL flag of the HCI device was reverted to ensure
the device is first closed before making it available again.

Due to hci_dev_close checking for HCI_USER_CHANNEL being set on the
device it was never really closed and was kept opened. We're now
calling hci_dev_do_close directly to make sure the device is correctly
closed and we keep the correct order to unset the flag on our device
object.

Signed-off-by: Simon Fels <simon.fels@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>