ixgbe: Make ixgbe_fc_autoneg return void and always set current_mode

This change makes it so that ixgbe_fc_autoneg is a void and always sets the
current_mode.  Previously if the link was down we would return an error,
however there is no harm in simply treating a link down case as a case in
which autoneg simply failed.  This allows us to rely on the return value of
the ixgbe_fc_enable call now since there should be no cases where it
returns an error that would normally be ignored.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>