Bluetooth: Convert Pair Device to use cmd_complete callback

This patch converts the Pair Device mgmt command to use the new
cmd_complete callback for pending mgmt commands. The already existing
pairing_complete() function is exactly what's needed and doesn't need
changing.

In addition to getting the return parameters always right this patch
actually fixes a reference counting bug and memory leak with the
hci_conn that's attached to the pending mgmt command - something that
would occur when powering off or unplugging the adapter while pairing is
in progress.

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