Bluetooth: Fix type in cpu_to_le conversion

Use struct hci_cp_write_def_link_policy to overcome sparse
warnigs below:

...
net/bluetooth/hci_event.c:633:21: warning: incorrect type in assignment
(different base types)
net/bluetooth/hci_event.c:633:21:    expected unsigned short [unsigned]
[assigned] [usertype] link_policy
net/bluetooth/hci_event.c:633:21:    got restricted __le16 [usertype] <noident>
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>