Bluetooth: Rename hci_find_ltk_by_addr to hci_find_ltk

Now that hci_find_ltk_by_addr is the only LTK lookup function there's no
need to keep the long name anymore. This patch shortens the function
name to simply hci_find_ltk.

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