bonding: remove __get_first_port()

Currently we have only one user of it, so it's kind of useless and just
obfusicates things.

Remove it and move the logic to the only user -
bond_3ad_state_machine_handler().

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>