pwm: stm32: Initialize raw local variables

This removes build warning when COMPILE_TEST=y and MFD_STM32_TIMERS=n
in drivers/pwm/pwm-stm32.c. In function 'stm32_pwm_capture' 'raw_prd' and
'raw_dty' may be used uninitialized in this function
[-Wmaybe-uninitialized]

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>