serial: make 8250 timeout use the specified IRQ handler

The current 8250 timeout code duplicates the code path in
serial8250_default_handle_irq and then serial8250_handle_irq
i.e. reading iir, check for IIR_NO_INT, and then calling
serial8250_handle_port.

So the immediate thought is to replace the duplicated code
with a call to serial8250_default_handle_irq.

But this highlights a problem.  We let 8250 driver variants
use their own IRQ handler via specifying their own custom
->handle_irq, but in the event of a timeout, we ignore their
handler and implicitly run serial8250_default_handle_irq instead.

So, go through the struct to get ->handle_irq and call that,
which for most will still be serial8250_default_handle_irq.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/tty/serial/8250.c

@@ -1740,11 +1740,8 @@ static void serial_unlink_irq_chain(struct uart_8250_port *up)
 static void serial8250_timeout(unsigned long data)
 {
 	struct uart_8250_port *up = (struct uart_8250_port *)data;
-	unsigned int iir;
 
-	iir = serial_in(up, UART_IIR);
-	if (!(iir & UART_IIR_NO_INT))
-		serial8250_handle_port(up);
+	up->port.handle_irq(&up->port);
 	mod_timer(&up->timer, jiffies + uart_poll_timeout(&up->port));
 }