Null pointer dereference when BFS VM is powered off

The driver incorrectly uses sli3_ring on SLI-4 adapters

Use the correct ring structure based on sli_rev
Signed-off-by: NDick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: NJames Smart <james.smart@broadcom.com>
Tested-by: NRaphael Silva <raphasil@linux.vnet.ibm.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>