isci: enable BCN in sci_port_add_phy()

Ensure we enable receiving BCN's from the
hardware when adding phy to isci_port.
Otherwise if we get BCN before the port is
created we won't see any BCN

Signed-off-by: Maciej Trela <maciej.trela@intel.com>
Reported-by: Richard Boyd <richard.g.boyd@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>