i40e: change log messages and error returns

When VFs are being reset, there is a brief window of time when they
cannot be configured because they don't have a VSI to configure. If
a script is quick, it can fall through that window. To avoid
defenestration, log a useful error message and return -EAGAIN.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>