[BNX2]: Fix minor bug in bnx2_has_work().

It is more correct to get the status block from the bnx2_napi struct
instead of the bnx2 struct.  It happens that they are the same in this
case because we are using the first MSIX vector.

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