Bluetooth: Refactor connection request handling

The conditions for accepting an incoming connections are already
non-trivial and will become more so once a white list is added. This
patch breaks up the checks for when to reject the request by creating a
helper function for it.

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