igc: Refactor VLAN priority filtering code

The whole VLAN priority filtering code is implemented in igc_ethtool.c
and mixes logic from ethtool and core parts. This patch refactors it so
core logic is moved to igc_main.c, aligning the VLAN priority filtering
code organization with the MAC address filtering code.

This patch also takes the opportunity to add some log messages to ease
debugging.

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>