igc: Change adapter->nfc_rule_lock to mutex

This patch changes adapter->nfc_rule_lock type from spin_lock to mutex
so we avoid unnecessary busy waiting on lock contention.

A closer look at the execution context of NFC rule API users shows that
all of them run in process context. The API users are: ethtool ops,
igc_configure(), called when interface is brought up by user or reset
workequeue thread, igc_down(), called when interface is brought down,
and igc_remove(), called when driver is unloaded.

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>