Bluetooth: controller: Fix incorrect conditional compilation

Fix incorrect conditional compilation that caused compile
error when lll_scan structure did not contain connection
context included which is needed only for central role.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>

subsys/bluetooth/controller/ll_sw/ull_scan.c

@@ -418,7 +418,7 @@ static u8_t disable(u16_t handle)
 		return BT_HCI_ERR_CMD_DISALLOWED;
 	}
 
-#if defined(CONFIG_BT_CONN)
+#if defined(CONFIG_BT_CENTRAL)
 	if (scan->lll.conn) {
 		return BT_HCI_ERR_CMD_DISALLOWED;
 	}