ixgbe: DCB: credit max only needs to be gt TSO size for 82598

The maximum credits per traffic class only needs to be greater
then the TSO size for 82598 devices. The 82599 devices do not
have this requirement so only do this test for 82598 devices.

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>