The Data sheet says u can only do it by parallel programing not with isp :

"This device is configured at the factory to operate using 6 clock periods per machine cycle, referred to in this datasheet as “6 clock mode”. (This yields performance equivalent to twice that of standard 80C51 family devices). It may be optionally configured on
commercially-available EPROM programming equipment to operate at 12 clocks per machine cycle, referred to in this datasheet as “12 clock mode”. Once 12 clock mode has been configured, it cannot be changed back to 6 clock mode."

in order to do it u must chane the 3rd config byte t0 0x80