serial: only use PNP IRQ if it's valid

"Luming Yu" <luming.yu@gmail.com> says:

  There is a "ttyS1 irq is -1" problem observed on tiger4 which cause the
  serial port broken.

  It is because that there is __no__ ACPI IRQ resource assigned for the
  serial port.  So the value of the IRQ for the port is never changed since it
  got initialized to -1.

If PNP supplies a valid IRQ, use it.  Otherwise, leave port.irq == 0, which
means "no IRQ" to the serial core.
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Yu Luming <luming.yu@intel.com>
Acked-by: Matthew Wilcox <matthew@wil.cx>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/8250_pnp.c

@@ -439,6 +439,7 @@ serial_pnp_probe(struct pnp_dev *dev, const struct pnp_device_id *dev_id)
 	}
 
 	memset(&port, 0, sizeof(struct uart_port));
+	if (pnp_irq_valid(dev, 0))
 		port.irq = pnp_irq(dev, 0);
 	if (pnp_port_valid(dev, 0)) {
 		port.iobase = pnp_port_start(dev, 0);