i40e: Fix comment for ethtool diagnostic link test

The existing comment is incorrect.  Add new comment to point out that the
PF reset does not affect link but if the reset is changed to a different
type that does affect link then the link test would need to be moved to
before the reset.

Change-ID: I28d786f46e9465860babdee61c1dba51016464df
Reported-by: Jeremiah Kyle <jeremiah.kyle@intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>