S2io: fix statistics flush after a MTU change

On s2io driver, when you change the interface MTU, it invokes a card
reset, which flush some statistics.  This patch solves this problem, and
also set the net_device->stats as the default statistics structure,
instead of s2io_nic->stats.

To do that, s2io_nic->stats turned into a staging area, where is saved
statistics of the last hardware statistics query. So, the difference
between the current hardware statistics and s2io_nic->stats, is the
value that should be summed up, in order to get the correct statistics
value, even after a reset.

Signed-off-by: Breno Leitao <leitao@linux.vnet.ibm.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>