Bluetooth: Add le_addr_type() helper function

The mgmt code needs to convert from mgmt/L2CAP address types to HCI in
many places. Having a dedicated helper function for this simplifies
code by shortening it and removing unnecessary 'addr_type' variables.

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