Bluetooth: controller: Fix compile error when LE_ENC is disabled

Fixes the following compile error when CONFIG_BT_CTLR_LE_ENC
is disabled:
subsys/bluetooth/controller/ll_sw/ctrl.c: In function
'isr_rx_conn_pkt_ctrl':
subsys/bluetooth/controller/ll_sw/ctrl.c:2613:29: error:
'LLCP_ENCRYPTION' undeclared (first use in this function)
         (conn->llcp_type != LLCP_ENCRYPTION)) ||
                             ^~~~~~~~~~~~~~~
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>