bnx2: Fix bug when saving statistics.

This fixes the problem of dropping the carry when adding 2 32-bit values.
Switch to use array indexing for better readability.

Reported by and fix provided by Patrick Rabau.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>