Bluetooth: Ignore SMP data on non-LE links

The SMP CID is only defined for LE transports. Instead of returning an
error from smp_sig_channel() in this case (which would cause a
disconnection) just return 0 to ignore the data, which is consistent
with the behavior for other unknown CIDs.

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