Bluetooth: Rename HCI_DEBUG_KEYS to HCI_KEEP_DEBUG_KEYS

We're planning to add a flag to actively use debug keys in addition to
simply just accepting them, which makes the current generically named
DEBUG_KEYS flag a bit confusing. Since the flag in practice affects
whether the kernel keeps debug keys around or not rename it to
HCI_KEEP_DEBUG_KEYS.

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