Bluetooth: Move New LTK store hint evaluation into mgmt_new_ltk

It's simpler (one less if-statement) to just evaluate the appropriate
value for store_hint in the mgmt_new_ltk function than to pass a boolean
parameter to the function. Furthermore, this simplifies moving the mgmt
event emission out from hci_add_ltk in subsequent patches.

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