bnx2x: Prevent null pointer dereference on error flow

If debug message is open and bnx2x_vfop_qdtor_cmd() were to fail,
the resulting print would have caused a null pointer dereference.

Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>