Bluetooth: Set conn->link_policy in hci_conn_add

This allows slaves to request sniff mode too when
idle_timeout is non-zero.  Otherwise, slaves connections
were not getting their link_policy initialized and
sniff mode requests would never be sent.

Change-Id: I3965e509ab03a9e00858415f9100fa4038d1c18d
Signed-off-by: Mike J. Chen <mjchen@google.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>