stm32/led: Support PWM output without TIM3.

For example, the STM32WB55 doesn't have TIM3 but can still drive LEDs using
PWM on other timers.

ports/stm32/led.c

@@ -141,9 +141,11 @@ STATIC void led_pwm_init(int led) {
         case 2:
             __TIM2_CLK_ENABLE();
             break;
+        #if defined(TIM3)
         case 3:
             __TIM3_CLK_ENABLE();
             break;
+        #endif
         default:
             assert(0);
     }