Current pm_genpd_add_subdomain() will allow duplicated link between
master and slave domain.  This patch fixed it.

Because when current pm_genpd_add_subdomain() checks whether the link
between the master and slave generic PM domain already exists,
slave_links instead of master_links of master domain is used.
Signed-off-by: NHuang Ying <ying.huang@intel.com>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>