bnx2x: consistent statistics after internal driver reload

Currently bnx2x statistics are reset by inner driver reload, e.g. by MTU
change. This patch fixes this issue - from now on statistics should only
be reset upon device closure.
Thanks to Michal Schmidt <mschmidt@redhat.com> for his initial patch
regarding this issue.

Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>