bonding: make ad_agg_selection_logic() use bond_for_each_slave()

Convert all instances of

for (agg = __get_first_agg(); agg; agg = __get_next_port)

to the standard bond_for_each_slave(). Also, remove the useless checks
before calling bond_3ad_set_carrier() - if we have something NULL - it
would fire long ago, in __get_first/next_port(), per example.

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>