igc: Remove IGC_MAC_STATE_QUEUE_STEERING

The IGC_MAC_STATE_QUEUE_STEERING bit in mac_table[i].state is
utilized to indicate that frames matching the filter are assigned to
mac_table[i].queue. This bit is not strictly necessary since we can
convey the same information as follows: queue == -1 means queue
assignment is disabled, otherwise it is enabled.

In addition to make the code simpler, this change fixes some awkward
situations where we pass a complete misleading 'queue' value such as in
igc_uc_sync().

So this patch removes IGC_MAC_STATE_QUEUE_STEERING and also takes the
opportunity to improve the igc_add_mac_filter documentation.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>