Bluetooth: Allow l2cap_chan_check_security() to be used for LE links.

With connection oriented L2CAP channels some code paths will be shared
with BR/EDR links. It is therefore necessary to allow the
l2cap_chan_check_security function to be usable also for LE links in
addition to BR/EDR ones. This means that smp_conn_security() needs to be
called instead of hci_conn_security() in the case of an LE link.

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