bonding: make ad_port_selection_logic() use bond_for_each_slave()

Currently, ad_port_selection_logic() uses

for (aggregator = __get_first_agg(port); aggregator;
     aggregator = __get_next_agg(aggregator)) {

construct, however it's suboptimal, difficult to read and understand.

Change it to a standard bond_for_each_slave(), so that we won't need
__get_first/next_agg() and have it more readable.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>