Bluetooth: L2CAP: Fix not checking l2cap_chan security level

l2cap_check_enc_key_size shall check the security level of the
l2cap_chan rather than the hci_conn since for incoming connection
request that may be different as hci_conn may already been
encrypted using a different security level.

Fixes: 522e9ed1 ("Bluetooth: l2cap: Check encryption key size on incoming connection")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>