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>