pwm: lpss: Move clk_rate check to ->probe()

There is no need to check each time if the clk_rate defined or not when we call
pwm_lpss_config(). Move the check to ->probe() instead.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>