serial: fixup /proc/tty/driver/serial after proc_fops conversion

"struct tty_driver *" lies in m->private not in v which is
SEQ_TOKEN_START which is 1 which is enough to trigger NULL dereference
next line:

	BUG: unable to handle kernel NULL pointer dereference at 000000ad
	IP: [<c040d689>] uart_proc_show+0xe/0x2b0

Noticed by Linus.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/serial_core.c

@@ -1765,7 +1765,7 @@ static void uart_line_info(struct seq_file *m, struct uart_driver *drv, int i)
 
 static int uart_proc_show(struct seq_file *m, void *v)
 {
-	struct tty_driver *ttydrv = v;
+	struct tty_driver *ttydrv = m->private;
 	struct uart_driver *drv = ttydrv->driver_state;
 	int i;