e1000e: Set InterruptThrottleRate to default when invalid value used

During module load, seting the InterruptThrottleRate parameter to an
invalid value would result in the itr/itr_setting pair being set to
unexpected values which would result in poor performance.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>