tg3: don't clear stats while tg3_close

Now tg3 NIC's stats will be cleared after ifdown/ifup. bond_get_stats traverse
its salves to get statistics,cumulative the increment.If a tg3 NIC is added to
bonding as a slave,ifdown/ifup will cause bonding's stats become tremendous value
(ex.1638.3 PiB) because of negative increment.

Fixes: 92feeabf ("tg3: Save stats across chip resets")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>