How to prescale HFPERCLK in EFR32FG14
You can't define them, they are already defined for you.
In fact, while it looks like they are ratios, they are the actual Nlow and Nhigh numbers.
For Standard Mode I2C, duty of 50%:50% or 1:1 is needed to reach maximum speed of 100kHz.
For Fast Mode I2C, duty of 16:9 is needed to reach maximum speed of 400kHz.
Since the MCU cannot do 16:9 but 6:3 it cannot reach maximum speed of 400kHz with the slowest rise time allowed by the I2C specs.