igc: Remove duplicate code in MAC filtering logic

This patch does a code refactoring in the MAC address filtering logic to
get rid of some duplicate code.

IGC driver has two functions to add MAC address filters that are pretty
much the same: igc_add_mac_filter() and igc_add_mac_filter_flags(). The
only difference is that the latter allows the callee to specify the
'flags' parameter while the former has it hard coded as zero. The same
rationale applies to filter deletion counterparts.

So this patch refactors igc_add_mac_filter() and igc_del_mac_filter() so
they handle the 'flags' parameters, removes the _flags() functions, and
fixes callees accordingly.

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>