serial: 8250_lpss: get IRQ via pci_irq_vector()

Instead of a direct assignment use pci_irq_vector() call as it's done for the
other case.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>