Bluetooth: Fix canceling RPA expiry timer

The RPA expiry timer is only initialized inside mgmt.c when we receive
the first command from user space. This action also involves setting the
HCI_MGMT flag for the first time so that flag acts as a good indicator
of whether the delayed work variable can be touched or not. This patch
fixes hci_dev_do_close to first check the flag.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>