DMA clock has to be enabled, too

src/main.c

@@ -22,6 +22,7 @@ static void clock_setup(void)
 	/* Enable clocks for peripherals. */
 	rcc_periph_clock_enable(RCC_USART2);
 	rcc_periph_clock_enable(RCC_ADC1);
+	rcc_periph_clock_enable(RCC_DMA);
 	rcc_periph_clock_enable(RCC_TIM21);
 }