pwm: sti: Ensure same period values for all channels

ST PWM IP shares the same clock prescaler across all the PWM
channels. Hence configuration requests which change the period
will affect all the channels. Do not allow period changes which
will stomp period settings of the already configured channels.

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>