[adc] replace #ifdef USE_ADC_x by #if USE_ADC_x

this makes it possible to explicitly disable them with -DUSE_ADC_x=0

sw/airborne/mcu.c

@@ -44,7 +44,7 @@
 #if defined USE_I2C0  || defined USE_I2C1  || defined USE_I2C2
 #include "mcu_periph/i2c.h"
 #endif
-#if defined USE_ADC
+#if USE_ADC
 #include "mcu_periph/adc.h"
 #endif
 #ifdef USE_USB_SERIAL
@@ -104,7 +104,7 @@ void mcu_init(void) {
 #ifdef USE_I2C3
   i2c3_init();
 #endif
-#ifdef USE_ADC
+#if USE_ADC
   adc_init();
 #endif
 #ifdef USE_USB_SERIAL