There are two speed grades of AD9257: 40Msps version (maximum sample rate is 40MHz) and 65Msps version (maximum sample rate is 65MHz). Nothing needs to be set for the AD9257 to work at these sample rates.
Register 0xFF is self-clearing. It will always read back 0x00.
The AD9257 clock inputs can handle 80MHz, but you must use the clock frequency divider (Register 0x0B) to divide the clock frequency to an acceptable rate for AD9257. In the default state (divide-by-1), 80MHz clock is too fast for AD9257.