STM32 F401: Correct support for USART6 on this chip. From Freddie Chopin

arch/arm/src/stm32/stm32_uart.h

@@ -63,28 +63,24 @@
  ************************************************************************************/
 /* Make sure that we have not enabled more U[S]ARTs than are support by
  * the device.
- *
- * REVISIT:  These should be replaced with the CONFIG_STM32_HAVE_USARTn now generated
- * by the Kconfig file.  The following logic is not valid for the STM32F401 which
- * supports 3 USARTS:  USART1, USART2, and USART6.
  */
 
-#if STM32_NUSART < 8
+#if STM32_NUSART < 8 && !defined(CONFIG_STM32_HAVE_USART8)
 #  undef CONFIG_STM32_UART8
 #endif
-#if STM32_NUSART < 7
+#if STM32_NUSART < 7 && !defined(CONFIG_STM32_HAVE_USART7)
 #  undef CONFIG_STM32_UART7
 #endif
-#if STM32_NUSART < 6
+#if STM32_NUSART < 6 && !defined(CONFIG_STM32_HAVE_USART6)
 #  undef CONFIG_STM32_USART6
 #endif
-#if STM32_NUSART < 5
+#if STM32_NUSART < 5 && !defined(CONFIG_STM32_HAVE_USART5)
 #  undef CONFIG_STM32_UART5
 #endif
-#if STM32_NUSART < 4
+#if STM32_NUSART < 4 && !defined(CONFIG_STM32_HAVE_USART4)
 #  undef CONFIG_STM32_UART4
 #endif
-#if STM32_NUSART < 3
+#if STM32_NUSART < 3 && !defined(CONFIG_STM32_HAVE_USART3)
 #  undef CONFIG_STM32_USART3
 #endif
 #if STM32_NUSART < 2