ledc: fix reported error when div_param == 256

ledc: fix reported error when div_param == 256

Whenever bit_num and freq_hz are such that div_param == 256, the code would flag that as an error.
In fact, value 256 of div_param was a valid one (HW requires that div_param >= 1.0, when interpreted as a Q10.8 fixed point value).
Changed the if statements a bit to make the logic clearer, and added more descriptive error messages.
Also added missing error for the unimplemented case of low speed mode.