i40e: add doxygen comment for new mode parameter

A recent patch updated the signature for i40e_aq_set_switch_config() to
add a new parameter 'mode'. It forgot to document the parameter in the
doxygen function header comment. Add the parameter to the function
description now.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>