igc: Use num_tx_queues when iterating over tx_ring queue

Use num_tx_queues rather than the IGC_MAX_TX_QUEUES fixed number 4 when
iterating over tx_ring queue since instantiated queue count could be
less than 4 where on-line cpu count is less than 4.

Fixes: ec50a9d4

 ("igc: Add support for taprio offloading")
Signed-off-by: Toshiki Nishioka <toshiki.nishioka@intel.com>
Signed-off-by: Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>
Tested-by: Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>
Acked-by: Sasha Neftin <sasha.neftin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>