igc: Limit netdev_tc calls to MQPRIO

Limit netdev_tc calls to MQPRIO. Currently these calls are made in
igc_tsn_enable_offload() and igc_tsn_disable_offload() which are used by
TAPRIO and ETF as well. However, these are only required for MQPRIO.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>