tg3: Fix INTx fallback when MSI fails

MSI setup changes the value of irq_vec in struct tg3 *tp.
This attribute must be taken into account and restored before
we try to do a new request_irq for INTx fallback.

In powerpc, the original code was leading to an EINVAL return within
request_irq, because the driver was trying to use the disabled MSI
virtual irq number instead of tp->pdev->irq.
Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>