Bluetooth: L2CAP: Fix not responding with L2CAP_CR_LE_ENCRYPTION

Depending on the security set the response to L2CAP_LE_CONN_REQ shall be
just L2CAP_CR_LE_ENCRYPTION if only encryption when BT_SECURITY_MEDIUM
is selected since that means security mode 2 which doesn't require
authentication which is something that is covered in the qualification
test L2CAP/LE/CFC/BV-25-C.

Link: https://github.com/bluez/bluez/issues/1270


Fixes: 27e2d4c8 ("Bluetooth: Add basic LE L2CAP connect request receiving support")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>