Bluetooth: Make inquiry_cache_flush non-static

In order to use HCI request framework in start_discovery, we'll need
to call inquiry_cache_flush in mgmt.c. Therefore, this patch adds the
hci_ prefix to inquiry_cache_flush and makes it non-static.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>