bluetooth: Fix L2CAP CoC response code if LTK is present

If an LTK or an STK is available and encryption is required
(LE security mode 1) but encryption is not enabled, the
service request shall be rejected with the error code
"Insufficient Encryption".

This is affecting L2CAP/LE/CFC/BV-25-C and L2CAP/ECFC/BV-32-C
qualification test cases.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>