USB: serial: f81534: 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: 3aacac02 ("USB: serial: f81534: add high baud rate support")
Cc: stable@vger.kernel.org      # 4.16
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/f81534.c

@@ -536,9 +536,6 @@ static int f81534_submit_writer(struct usb_serial_port *port, gfp_t mem_flags)
 
 static u32 f81534_calc_baud_divisor(u32 baudrate, u32 clockrate)
 {
-	if (!baudrate)
-		return 0;
-
 	/* Round to nearest divisor */
 	return DIV_ROUND_CLOSEST(clockrate, baudrate);
 }
@@ -568,9 +565,14 @@ static int f81534_set_port_config(struct usb_serial_port *port,
 	u32 baud_list[] = {baudrate, old_baudrate, F81534_DEFAULT_BAUD_RATE};
 
 	for (i = 0; i < ARRAY_SIZE(baud_list); ++i) {
-		idx = f81534_find_clk(baud_list[i]);
+		baudrate = baud_list[i];
+		if (baudrate == 0) {
+			tty_encode_baud_rate(tty, 0, 0);
+			return 0;
+		}
+
+		idx = f81534_find_clk(baudrate);
 		if (idx >= 0) {
-			baudrate = baud_list[i];
 			tty_encode_baud_rate(tty, baudrate, baudrate);
 			break;
 		}