drivers: serial: stm32: fix race condition when both APIs enabled

Fix a bug where if CONFIG_UART_ASYNC_API was enabled in the config, the
interrupt flags would get cleared after the interrupt driven API callback
finished executing, causing data loss if data happened to arrive after the
callback but before the flags were cleared.

Signed-off-by: Kamil Krzyżanowski <kamnxt@kamnxt.com>