i40e: Fix ethtool offline test

If the system administrator is requesting an offline diagnostic test using
'ethtool -t' then we should, you know, actually take the device offline
before doing the testing.

Change-ID: I6afa1cbfcc821c9ab6e6f47ed4d8dc2d8dd20e82
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>