pwm: sti: Fix PWM prescaler handling

This patch fixes the pwm driver to write the complete 8 bits of
the prescaler value to the PWM Control register.

Signed-off-by: Ajit Pal Singh <ajitpal.singh@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>