bnx2x: Fix VF statistics

After a VF performs load/unload its statistics become corrupt -
we now zero the statistics structures upon a VF device load.

Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>