cnic: Fix occasional NULL pointer dereference during reboot.

We register with bnx2x before we allocate ctx_tbl structure, so it is
possible for bnx2x to call cnic_ctl before the structure is allocated.
This can sometimes cause NULL pointer dereference of cp->ctx_tbl.  We
fix this by adding simple checking for valid state before proceeding.
The cnic_ctl call is RCU protected so we don't have to deal with race
conditions.

Because of the additional checking, we need to finish the shutdown
before clearing the CNIC_UP flag.

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