igc: Integrate flex filter into ethtool ops

Use the flex filter mechanism to extend the current ethtool filter
operations by intercoperating the user data. This allows to match
eight more bytes within a Ethernet frame in addition to macs, ether
types and vlan.

The matching pattern looks like this:

 * dest_mac [6]
 * src_mac [6]
 * tpid [2]
 * vlan tci [2]
 * ether type [2]
 * user data [8]

This can be used to match Profinet traffic classes by FrameID range.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>