[STM32F3] Fix for pmw period change

The new period needs to be saved before the duty cycle is updated as
the period is used in pwmout_write function.

Also presclaer shall better be initiliazed properly.

hal/targets/hal/TARGET_STM/TARGET_STM32F3/pwmout_api.c

@@ -69,6 +69,7 @@ void pwmout_init(pwmout_t* obj, PinName pin)
     obj->pin = pin;
     obj->period = 0;
     obj->pulse = 0;
+    obj->prescaler = 1;
 
     pwmout_period_us(obj, 20000); // 20 ms per default
 }
@@ -188,12 +189,12 @@ void pwmout_period_us(pwmout_t* obj, int us)
         error("Cannot initialize PWM");
     }
 
-    // Set duty cycle again
-    pwmout_write(obj, dc);
-
     // Save for future use
     obj->period = us;
 
+    // Set duty cycle again
+    pwmout_write(obj, dc);
+
     __HAL_TIM_ENABLE(&TimHandle);
 }