ixgbe: DCB X540 devices support max traffic class of 4

X540 devices can only support up to 4 traffic classes and
guarantee a "lossless" traffic class on some platforms.
This patch sets the X540 devices to initialize a max
traffic class value of 4 at probe time.

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>