bonding: Remove unneeded list_empty checks.

Some places iterate over the checked list right after the check
itself, so even if the list is empty, the list_for_each_xxx
iterator will make everything right by himself.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>