clock_prescale_get()
Gets and returns the clock prescaler register setting. The return type is clock_div_t.
For device with XTAL Divide Control Register (XDIV), return can actually range from 1 to 129. Care should be taken has the return value could differ from the typedef enum clock_div_t. This should only happen if clock_prescale_set was previously called with a value other than those defined by clock_div_t.