igc: block setting preemptible traffic class in taprio

Since preemptible tc implementation is not ready yet, block it from being
set in taprio. The existing code already blocks it in mqprio.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>