Bluetooth: Update smp_confirm to return a response code

Now that smp_confirm() is called "inline" we can have it return a
response code and have the sending of it be done in the shared place for
command handlers. One exception is when we're entering smp.c from mgmt.c
when user space responds to authentication, in which case we still need
our own code to call smp_failure().

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