bnx2: Fix netpoll crash.

The bnx2 driver calls netif_napi_add() for all the NAPI structs during
->probe() time but not all of them will be used if we're not in MSI-X
mode.  This creates a problem for netpoll since it will poll all the
NAPI structs in the dev_list whether or not they are scheduled, resulting
in a crash when we access structure fields not initialized for that vector.

We fix it by moving the netif_napi_add() call to ->open() after the number
of IRQ vectors has been determined.

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