igc: Fix 'sw_idx' type in struct igc_nfc_rule

The 'sw_idx' field from 'struct igc_nfc_rule' is u16 type but it is
assigned an u32 value in igc_ethtool_init_nfc_rule(). This patch changes
'sw_idx' type to u32 so they match. Also, it makes more sense to call
this field 'location' since it holds the NFC rule location.

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>