Bluetooth: Fix doing some useless casts when receiving MGMT commands

Every command handler of mgmt does a cast to the command structure
so it can properly interpreted. So we can avoid that cast if we
make those functions receive a void * directly.
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>