net-forcedeth: fix possible stats inaccuracies on 32b hosts

The software stats are updated from BH, this change ensures that 32b
UP hosts use appropriate protection.

Tested:
  - HW/SW stats consistent with pktgen (in particular tx=rx)
  - HW/SW stats consistent when tx/rx offloads disabled
  - no problem with+without lockdep (SMP 16-way)

Signed-off-by: David Decotigny <david.decotigny@google.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>