stm32/stm32_it: Guard UART7_IRQHandler with check for UART7 define.

All STM32 with a UART7 also have a UART8 and vice versa, but this change
improves readability and allows for them to be independent in the future.

ports/stm32/stm32_it.c

@@ -735,7 +735,7 @@ void USART6_IRQHandler(void) {
     IRQ_EXIT(USART6_IRQn);
 }
 
-#if defined(UART8)
+#if defined(UART7)
 void UART7_IRQHandler(void) {
     IRQ_ENTER(UART7_IRQn);
     uart_irq_handler(7);