bnx2x: pfc statistics counts pfc events twice

When pfc statistics were counted, the delta change from last count
was summed twice. This fixes the 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>