cnic: Fix crash during bnx2x MTU change.

cnic_service_bnx2x() irq handler can be called during chip reset from
MTU change.  Need to check that the cnic's device state is up before
handling the irq.

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