bonding: fix link detect in non-interrupt mode

bonding: fix link detect in non-interrupt mode

Stopping then re-starting a bond interface containing slaves that
used polling for link detection caused the bond to think all slave
links were down and inactive.

Move the start of the polling for link from slave_add() to
bond_ethdev_start() and in bond_ethdev_stop() make sure we clear
the last_link_status of the slaves.

Fixes: a45b288ef21a ("bond: support link status polling")

Signed-off-by: Nelson Escobar <neescoba@cisco.com>
Signed-off-by: John Daley <johndale@cisco.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>