Bluetooth: Prevent double register of suspend

Suspend notifier should only be registered and unregistered once per
hdev. Simplify this by only registering during driver registration and
simply exiting early when HCI_USER_CHANNEL is set.

Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: 359ee4f8 (Bluetooth: Unregister suspend with userchannel)
Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>