Merge pull request #6378 from haukepetersen/fix_stm32f4_hwrngtypo

cpu/stm32f4: fixed minor typo in the hwrng driver

cpu/stm32f4/periph/hwrng.c

@@ -56,4 +56,4 @@ void hwrng_read(uint8_t *buf, unsigned int num)
     periph_clk_dis(AHB2, RCC_AHB2ENR_RNGEN);
 }
 
-#endif /* CPUID_LEN */
+#endif /* RNG */