Bluetooth: Make is_identity_address a global function

There are more places that can take advantage of is_identity_address()
besides hci_core.c. This patch moves the function to hci_core.h and
gives it the appropriate hci_ prefix.

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