serial: 8250_pci: fix divide error bug if baud rate is 0

Since commit 21947ba6 ("serial: 8250_pci: replace switch-case by
formula"), the 8250 driver crashes in the byt_set_termios() function
with a divide error. This is caused by the fact that a baud rate of 0 (B0)
is not handled properly. Fix it by falling back to B9600 in this case.
Signed-off-by: David Müller <d.mueller@elsoft.ch>
Fixes: 21947ba6 ("serial: 8250_pci: replace switch-case by formula")
Cc: stable@vger.kernel.org
Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_pci.c

@@ -1377,6 +1377,9 @@ byt_set_termios(struct uart_port *p, struct ktermios *termios,
 	unsigned long m, n;
 	u32 reg;
 
+	/* Gracefully handle the B0 case: fall back to B9600 */
+	fuart = fuart ? fuart : 9600 * 16;
+
 	/* Get Fuart closer to Fref */
 	fuart *= rounddown_pow_of_two(fref / fuart);