ice: SW DCB, report correct max TC value

lldpad is using the value reported in the DCB config for
max_tc as the max allowed number of TCs, not the current
max.  ICE driver was reporting it as current maximum TC.

Change DCB_NL function to report maximum TC allowed by
this device.

Signed-off-by: Dave Ertman <david.m.ertman@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>