i40e: fix clearing link masks in i40e_get_link_ksettings

This fixes two issues in i40e_get_link_ksettings.  It adds calls to
ethtool_link_ksettings_zero_link_mode to make sure advertising and
supported link masks are cleared before we start setting bits in them.

This also replaces some funky bit manipulations with a much nicer call
to ethtool_link_ksettings_del_link_mode when removing link modes.

Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>