stm32f4: usart: Fix wrong usart4 alternative function number
stm32f4: usart: Fix wrong usart4 alternative function number

It appears as name of `usart4` in `/dev` directory though it's actually
not usart but uart. Keep it in mind since I'm not going to fix it.

onkwon/yaos,onkwon/yaos,onkwon/yaos