serial: stm32: fix dropped `UART_TX_DONE` events

Fix dropped `UART_TX_DONE` events when poll out and async APIs are used
on the same port. Clearing `tx_poll_stream_on` is required to prevent
`uart_stm32_isr` from prematurely clearing the TC status bit.

Signed-off-by: Jordan Yates <jordan@embeint.com>
1 file changed