pwm: stm32-lp: Remove pwm_is_enabled() check before calling pwm_disable()

The same checking is done by the implementation of pwm_disable().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>