drivers: serial: uart_stm32.c: RxDataFlush on async_rx_enable

When enabling async RX the first time after boot, there is an
additional byte received with the first RX_DATA_RDY event,
which seems to be caused by the RX data not being flushed before
enabling the UART RX DMA.

Adding a request to flush the RX data register before enabling
the RX DMA, solves the issue.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>