Bluetooth: Add controller side link key clearing to hci_init_req

The controller may have link keys in its own memory and these keys could
be used for secure connections. However, since the interface to access
these keys doesn't provide information about the key types (which would
be needed to infer the level of security each key provides) using these
keys is rather useless. Therefore, simply clear the controller side list
in the initialization procedure.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>