tty: serial: fsl_lpuart: fix lpuart32_poll_get_char

The watermark is set to 1, so we need to input two chars to trigger RDRF
using the original logic. With the new logic, we could always get the
char when there is data in FIFO.
Suggested-by: NFugang Duan <fugang.duan@nxp.com>
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20200929095509.21680-1-peng.fan@nxp.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/fsl_lpuart.c

@@ -680,7 +680,7 @@ static void lpuart32_poll_put_char(struct uart_port *port, unsigned char c)
 
 static int lpuart32_poll_get_char(struct uart_port *port)
 {
-	if (!(lpuart32_read(port, UARTSTAT) & UARTSTAT_RDRF))
+	if (!(lpuart32_read(port, UARTWATER) >> UARTWATER_RXCNT_OFF))
 		return NO_POLL_CHAR;
 
 	return lpuart32_read(port, UARTDATA);