igb: make certain to reassign legacy interrupt vectors after reset

This change corrects an issue that will cause false hangs when using either
82575 or 82580 in legacy interrupt mode.  The issue is caused when there is
a slow traffic flow and an "ethtool -r" is executed while using legacy or
MSI interrupts.  MSI-X is not affected by this issue due to the fact that
we were already reconfiguring the vectors after reset.

If possible it would be best to push this for net-2.6 since it is resolving
a bug but if that is not possible then net-next-2.6 will be fine.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>