[TG3]: Set minimal hw interrupt mitigation.

Even though we do software interrupt mitigation
via NAPI, it still helps to have some minimal
hw assisted mitigation.

This helps, particularly, on systems where register
I/O overhead is much greater than the CPU horsepower.

For example, it helps on NUMA systems.  In such cases
the PIO overhead to disable interrupts for NAPI accounts
for the majority of the packet processing cost.  The
CPU is fast enough such that only a single packet is
processed by each NAPI poll call.

Thanks to Michael Chan for reviewing this patch.

Signed-off-by: David S. Miller <davem@davemloft.net>