igc: enable HW vlan tag insertion/stripping by default

This is enabled by default in other Intel drivers I've checked (e1000, e1000e,
iavf, igb and ice). Fixes an out-of-the-box performance issue when running
OpenWrt on typical mini-PCs with igc-supported Ethernet controllers and 802.1Q
VLAN configurations, as ethtool isn't part of the default packages and sane
defaults are expected.

In my specific case, with an Intel N100-based machine with four I226-V Ethernet
controllers, my upload performance increased from under 30 Mb/s to the expected
~1 Gb/s.

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Vitaly Lifshits <vitaly.lifshits@intel.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>