bnx2x: Prevent null pointer dereference in AFEX mode

The cnic module is responsible for initializing various bnx2x structs
via callbacks provided by the bnx2x module.
One such struct is the queue object for the FCoE queue.

If a device is working in AFEX mode and its configuration allows FCoE yet
the cnic module is not loaded, it's very likely a null pointer dereference
will occur, as the bnx2x will erroneously access the FCoE's queue object.

Prevent said access until cnic properly registers itself.

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>