ixgbe: fixup hard dependencies on supporting 8 traffic classes

This patch correctly configures DCB when less than 8 traffic classes
are available in hardware.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>