pci: change msi-x vector to 32bit

we are using 28bit pci (bus/dev/fn + 12 bits) as irq number, so the
cache for irq number should be 32 bit too.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>