pwm: sti: Only request clock rate when needed

In the original code the clock rate was only obtained during
initialisation; however, the rate may change between then and
its use.  This patch ensures the correct rate is acquired just
before use.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>