ixgbe: DCB, return max for IEEE traffic classes

Returning the max traffic classes on get requests simplifies
user space configurations because applications will know
explicitly how many traffic classes can be used.

Typical switch implementations use 2 or 3 traffic classes
so this not seen often today. And user space can learn
the number of traffic classes by return codes but this
allows user space to configure ixgbe correctly at the
start.

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>