Bluetooth: SSP: Support security level 4

Currently, error code `-ENOTSUP` will be
returned if start security with security
level 4.

For SC supported case, level 4 for ssp
should be supported.

Remove the code limitation to support
security level 4.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>

Author: lylezhu2012

subsys/bluetooth/host/classic/ssp.c

@@ -355,10 +355,6 @@ int bt_ssp_start_security(struct bt_conn *conn)
 		return -EBUSY;
 	}
 
-	if (conn->required_sec_level > BT_SECURITY_L3) {
-		return -ENOTSUP;
-	}
-
 	if (get_io_capa() == BT_IO_NO_INPUT_OUTPUT &&
 	    conn->required_sec_level > BT_SECURITY_L2) {
 		return -EINVAL;