forcedeth: msi interrupt fix

This patch fixes an issue with the suspend/resume cycle with msi
interrupts. See bugzilla number 10487 for more details. The fix is to
re-setup a private msi pci config offset field.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>