Bluetooth: Add 'Already Paired' error for Pair Device command

To make the behavior predictable when attempting to pair with a device
for which we already have a Link Key or Long Term Key, this patch adds a
new 'Already Paired' error which gets sent in such a scenario.

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