Bluetooth: Introduce hci_req helper to abort a connection

There are several different places needing to make sure that a
connection gets disconnected or canceled. The exact action needed
depends on the connection state, so centralizing this logic can save
quite a lot of code duplication.

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