i40e: use priv flags to control packet split

Ethtool priv flags implementation to enable or disable packet split, which
is a hardware feature that inspects headers and will put headers in a
separate DMA buffer from the payload data.  The driver was automatically
choosing to enable packet split in some cases and this gives the user the
ability to turn it off/on explicitly.

to query state:
ethtool --show-priv-flags ethx

to enable:
ethtool --set-priv-flags ethx packet-split on
to disable:
ethtool --set-priv-flags ethx packet-split off

Why would anyone want this?
	Because some environments benefit from header/data split in the receive
	buffer, and the driver defaults to one or the other depending on
	environment/kernel parameters.

Why didn't you implement a generic ethtool control for this feature?
	Because Intel hardware is the only hardware that supports header/data
	split.

Change-ID: I803121e1eecc9ccb2884031fd85dd1110b3af66d
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>