USB: ftdi_sio: Quiet sparse noise about using plain integer was NULL pointer

Pointers should not be compared to plain integers.
Quiets the sparse warning:
warning: Using plain integer as NULL pointer
Signed-off-by: NYing Xue <ying.xue@windriver.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/ftdi_sio.c

@@ -2106,7 +2106,7 @@ static void ftdi_set_termios(struct tty_struct *tty,
 
 	cflag = termios->c_cflag;
 
-	if (old_termios == 0)
+	if (!old_termios)
 		goto no_skip;
 
 	if (old_termios->c_cflag == termios->c_cflag