serial: imx: get rid of unbounded busy-waiting loop

imx_set_termios(): remove busy-waiting "drain Tx FIFO" loop. Worse
yet, it was potentially unbounded wait due to RTS/CTS (hardware)
handshake.

Let user space ensure draining is done before termios change, if
draining is needed in the first place.
Signed-off-by: Sergey Organov <sorganov@gmail.com>
Link: https://lore.kernel.org/r/1567017475-11919-2-git-send-email-sorganov@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -1644,7 +1644,7 @@ imx_uart_set_termios(struct uart_port *port, struct ktermios *termios,
 	uart_update_timeout(port, termios->c_cflag, baud);
 
 	/*
-	 * disable interrupts and drain transmitter
+	 * disable interrupts
 	 */
 	old_ucr1 = imx_uart_readl(sport, UCR1);
 	imx_uart_writel(sport,
@@ -1652,9 +1652,6 @@ imx_uart_set_termios(struct uart_port *port, struct ktermios *termios,
 			UCR1);
 	imx_uart_writel(sport, old_ucr2 & ~UCR2_ATEN, UCR2);
 
-	while (!(imx_uart_readl(sport, USR2) & USR2_TXDC))
-		barrier();
-
 	/* then, disable everything */
 	imx_uart_writel(sport, old_ucr2 & ~(UCR2_TXEN | UCR2_RXEN | UCR2_ATEN), UCR2);