BugFix:Prevent drv_led_pwm passing a value of 0 to px4_arch_configgpio

This prevents the meta value of 0 => not used from being passed to
  px4_arch_configgpio. As this would map to PORTA|PIN0 and is not
  the intended configuration.

src/drivers/stm32/drv_led_pwm.cpp

@@ -167,10 +167,13 @@ static void led_pwm_timer_init_timer(unsigned timer)
 static void
 led_pwm_channel_init(unsigned channel)
 {
+	/* Only initialize used channels */
+
+	if (led_pwm_channels[channel].timer_channel) {
+
 		unsigned timer = led_pwm_channels[channel].timer_index;
 
 		/* configure the GPIO first */
-
 		px4_arch_configgpio(led_pwm_channels[channel].gpio_out);
 
 		/* configure the channel */
@@ -195,6 +198,7 @@ led_pwm_channel_init(unsigned channel)
 			rCCER(timer) |= GTIM_CCER_CC4E;
 			break;
 		}
+	}
 }
 
 int