USB: serial: f81232: fix division by zero on line-speed change

The driver leaves the line speed unchanged in case a requested speed is
not supported. Make sure to handle the case where the current speed is
B0 (hangup) without dividing by zero when determining the clock source.

Fixes: 268ddb5e ("USB: serial: f81232: add high baud rate support")
Cc: stable@vger.kernel.org      # 5.2
Cc: Ji-Ze Hong (Peter Hong) <hpeter@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/f81232.c

@@ -130,9 +130,6 @@ static u8 const clock_table[] = { F81232_CLK_1_846_MHZ, F81232_CLK_14_77_MHZ,
 
 static int calc_baud_divisor(speed_t baudrate, speed_t clockrate)
 {
-	if (!baudrate)
-		return 0;
-
 	return DIV_ROUND_CLOSEST(clockrate, baudrate);
 }
 
@@ -498,9 +495,14 @@ static void f81232_set_baudrate(struct tty_struct *tty,
 	speed_t baud_list[] = { baudrate, old_baudrate, F81232_DEF_BAUDRATE };
 
 	for (i = 0; i < ARRAY_SIZE(baud_list); ++i) {
-		idx = f81232_find_clk(baud_list[i]);
+		baudrate = baud_list[i];
+		if (baudrate == 0) {
+			tty_encode_baud_rate(tty, 0, 0);
+			return;
+		}
+
+		idx = f81232_find_clk(baudrate);
 		if (idx >= 0) {
-			baudrate = baud_list[i];
 			tty_encode_baud_rate(tty, baudrate, baudrate);
 			break;
 		}